Bringing GeoModelXML into GeoModel

This MR brings GeoModelXML from GeoModelATLAS to GeoModel.

The version of GeoModelXML that has brought in is a merge of the latest changes to the Athena version of GeoModelXml with the one in GeoModelATLAS.

In particular:

• latest Athena merged into GeoModelATLAS: atlas/athena@a9f5685c

• latest GeoModelATLAS changes picked: https://gitlab.cern.ch/atlas/geomodelatlas/GeoModelATLAS/-/commit/9818334ffb5466095d7f5b1e4a79cede2bb88951