[GEOT-4607] GML3 bindings either fail to parse 2.5D data (srsDimension=3), or silently drop the third ordinate

The changes are simple/mechanic, but I guess a quick review would not hurt.
The only "interesting" bit imho is searching for the srsDimension attribute in parent nodes, since typically it is found in the topmost geometry, not in the postlist or in the nested geometric elements