Change definitions in the UML model, not in the derived standard

[TatjanaKutzner]

Some edits have been done to definitions of packages, feature types, and attributes in the XSD files before publication of the CityGML GML Encoding Standard.
Since the definitions were originally defined in the UML model and derived from there in the XSD files, also edits should be done in the UML model. In this way the edits can be included in all documents and files by automatic derivation.

Commit aec5c51 includes these edits in the Conceptual Model and commit 64a716b in the Implementation Model.