support gml/3.2 namespace, nested elements in metadata

Solve #2977 - addresses several issues with the service mentioned:

* Namespace http://www.opengis.net/gml/3.2 as alternative to http://www.opengis.net/gml supported in all GML3 and relevant base classes.
* Detect GML geometry element by confirming that its child elements actually are known geometries
* support boundedBy on FeatureCollections
* parse nested metadata into an object structure, return flat text if the structure is not nested to maintain backwards compatibility.