Add gml_id as datatype

[ivbeg]

gml_id commonly used when GML (Geographic markup language) data converted to CSV/JSON datasets. It's defined as gml:id in the GML standard and represented as gml_id in datasets.

No such datatype in Wikidata. But it's common in datasets 774 occurences in OpenDataSoft metadata dataset and in Europe.

An idea is to investigate gml:id usage and to create new datatype for it.

[ivbeg]

From CSIRO AU wiki https://confluence.csiro.au/display/seegrid/Object+identifiers+in+GML#ObjectidentifiersinGML-gml:idattribute

gml:id is a standard GML attribute available on every object-with-identity. It has type="xs:ID" - i.e. it is a fragment identifier,
unique within document scope only, for internal cross-references. It is not useful by itself as a persistent unique identifier.

Looks like gml_id is gml:id and it's local identifier only, not an unique persistent id.