Duplicate initialisation message codes in NetCDFRead

[sadielbartholomew]

Whilst improving some docstrings in #183 I noticed that there were two cases where duplicate integer values were provided as codes in the _code0 dictionary, namely duplication on 200 and 201 with:

cfdm/cfdm/read_write/netcdf/netcdfread.py

Lines 35 to 36 in a9816e0

	"Bounds variable": 200,
	"bounds attribute": 201,

and

cfdm/cfdm/read_write/netcdf/netcdfread.py

Lines 53 to 54 in a9816e0

	"Tie points coordinate variable": 200,
	"Bounds tie points variable": 201,

where for all other keys in that dictionary, and indeed in the similar dict _code1, the values are unique numbers. Possibly this wasn't noticed before due to the ordering of the key-value pairs, which are ordered with the values strictly increasing except for the former pair of bounds-related cases which were out-of-place.

@davidhassell is this an issue or is there a reason for the duplication? Thanks.