Add 3 IDX-refXYZ-values to the standard for IDX up- and downstream compability

[merrycoder]

Problem description
When listing data originating from IDX sources is mapped to SwissRETS, the original triple-identifier-key refProperty, refHouse and refObject cannot be mapped, as SwissRETS only has a single referenceId field. In practice the 3 refXYZ-values are concatenated into the SwissREST referenceId. When further down the processing chain the original 3 refXYZ-values are needed (e.g. for RemCat leads forwarding) the values have to be extracted from the concatenated form, which is error prone.

Desired Solution
The 3 refXYZ-values are added as attributes in the following form:

properties->property->externalReference->refProperty
properties->property->externalReference->refHouse
properties->property->externalReference->refObject