Document accepted argument types for source parameters

[sadielbartholomew]

Equivalent to NCAS-CMS/cf-python#492 but note that here there are many more (in total ~50) classes which accept the source: parameter (since these get inherited downstream for cf-python which doesn't need to redefine them), so once we decide what to document for that parameter, consistently, we probably want to start by updating them here in cfdm:

$ pwd
/home/sadie/cfdm/cfdm
$ git grep "source:"
auxiliarycoordinate.py:            source: optional
bounds.py:            source: optional
cellmeasure.py:            source: optional
coordinatereference.py:            source: optional
core/abstract/container.py:            source: optional
core/abstract/parameters.py:            source: optional
core/abstract/parameters.py:        if source:
core/abstract/parametersdomainancillaries.py:            source: optional
core/abstract/parametersdomainancillaries.py:        if source:
core/abstract/properties.py:            source: optional
core/abstract/propertiesdata.py:            source: optional
core/abstract/propertiesdatabounds.py:            source: optional
core/bounds.py:            source: optional
core/cellmeasure.py:            source: optional
auxiliarycoordinate.py:            source: optional
bounds.py:            source: optional
cellmeasure.py:            source: optional
coordinatereference.py:            source: optional
core/abstract/container.py:            source: optional
core/abstract/parameters.py:            source: optional
core/abstract/parameters.py:        if source:
core/abstract/parametersdomainancillaries.py:            source: optional
core/abstract/parametersdomainancillaries.py:        if source:
core/abstract/properties.py:            source: optional
core/abstract/propertiesdata.py:            source: optional
core/abstract/propertiesdatabounds.py:            source: optional
core/bounds.py:            source: optional
core/cellmeasure.py:            source: optional
auxiliarycoordinate.py:            source: optional
bounds.py:            source: optional
cellmeasure.py:            source: optional
coordinatereference.py:            source: optional
core/abstract/container.py:            source: optional
core/abstract/parameters.py:            source: optional
core/abstract/parameters.py:        if source:
core/abstract/parametersdomainancillaries.py:            source: optional
core/abstract/parametersdomainancillaries.py:        if source:
core/abstract/properties.py:            source: optional
core/abstract/propertiesdata.py:            source: optional
core/abstract/propertiesdatabounds.py:            source: optional
core/bounds.py:            source: optional
core/cellmeasure.py:            source: optional
core/cellmethod.py:            source: optional
core/cellmethod.py:        if source:
core/constructs.py:            source: optional
core/coordinatereference.py:            source: optional
core/coordinatereference.py:        if source:
core/data/data.py:            source: *optional*
core/domain.py:            source: optional
core/domainaxis.py:            source:
core/field.py:            source: optional
count.py:            source: optional
data/abstract/compressedarray.py:            source: optional
data/abstract/raggedarray.py:            source: optional
data/data.py:            source: optional
data/gatheredarray.py:            source: optional
data/netcdfarray.py:            source: optional
data/raggedcontiguousarray.py:            source: optional
data/raggedindexedarray.py:            source: optional
data/raggedindexedcontiguousarray.py:            source: optional
data/subarray/abstract/subarray.py:            source: optional
data/subarray/abstract/subsampledsubarray.py:            source: optional
data/subarray/gatheredsubarray.py:            source: optional
data/subsampledarray.py:            source: optional
datum.py:            source: optional
dimensioncoordinate.py:            source: optional
domain.py:            source: optional
(cf-env) :)[14:20:08]cfdm>git grep "source:"
auxiliarycoordinate.py:            source: optional
bounds.py:            source: optional
cellmeasure.py:            source: optional
coordinatereference.py:            source: optional
core/abstract/container.py:            source: optional
core/abstract/parameters.py:            source: optional
core/abstract/parameters.py:        if source:
core/abstract/parametersdomainancillaries.py:            source: optional
core/abstract/parametersdomainancillaries.py:        if source:
core/abstract/properties.py:            source: optional
core/abstract/propertiesdata.py:            source: optional
core/abstract/propertiesdatabounds.py:            source: optional
core/bounds.py:            source: optional
core/cellmeasure.py:            source: optional
core/cellmethod.py:            source: optional
core/cellmethod.py:        if source:
core/constructs.py:            source: optional
core/coordinatereference.py:            source: optional
core/coordinatereference.py:        if source:
core/data/data.py:            source: *optional*
core/domain.py:            source: optional
core/domainaxis.py:            source:
core/field.py:            source: optional
count.py:            source: optional
data/abstract/compressedarray.py:            source: optional
data/abstract/raggedarray.py:            source: optional
data/data.py:            source: optional
data/gatheredarray.py:            source: optional
data/netcdfarray.py:            source: optional
data/raggedcontiguousarray.py:            source: optional
data/raggedindexedarray.py:            source: optional
data/raggedindexedcontiguousarray.py:            source: optional
data/subarray/abstract/subarray.py:            source: optional
data/subarray/abstract/subsampledsubarray.py:            source: optional
data/subarray/gatheredsubarray.py:            source: optional
data/subsampledarray.py:            source: optional
datum.py:            source: optional
dimensioncoordinate.py:            source: optional
domain.py:            source: optional
domainancillary.py:            source: optional
domainaxis.py:            source: optional
field.py:            source: optional
fieldancillary.py:            source: optional
index.py:            source: optional
interiorring.py:            source: optional
interpolationparameter.py:            source: optional
list.py:            source: optional
mixin/files.py:            source: optional
mixin/netcdf.py:            source: optional
mixin/propertiesdatabounds.py:            source: optional
nodecountproperties.py:            source: optional
partnodecountproperties.py:            source: optional
tiepointindex.py:            source: optional
$ git grep "source:" | wc -l
53