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

added 2 commits

• 60114b77 - (cmake) Install GeoModelXml and GMXPlugin a parts of GeoModelTools
• 6de897fb - (cmake) GeoModelXML: Clean unused code.

Compare with previous version

I have now modified the CMake configuration to install GeoModelXml and GMXPlugin as parts of GeoModelTools. That means that end users can pick GeoModelXml, GMXPlugin, and all other tools by merely using:

find_package( GeoModelTools )

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cc: @dellacqu @boudreau @tsulaia

The MR will be auto-merged when the CI pipeline will complete.

enabled an automatic merge when the pipeline for 6de897fb succeeds

merged

mentioned in commit 2d5f5ef5