PixelGeoModelXml package in master

Adding a package analogous to StripGeoModelXml but for ITkPixel detector in master.

The package includes:

• python/ITkPixelGeoModelConfig.py: python config for ITkPixelGeometry which configures PixelDetectorTool
• PixelDetectorTool: Athena tool to handle services & tools needed for building Pixel geometry (entry point of package)
• PixelDetectorFactory: Main routine to build the GeoModel geometry, and handle GeometryManager and DetectorManager
• PixelGmxInterface: GeoModelXml interface for Pixel
• ModuleTree: nested set of maps where user need only create the top-level item, a ModuleTree, and Modules

The package is incomplete in the sense that the alignment options have not yet been added to the gmx interface or the detector factory.

The next steps are to run tests with the ITKLayouts branch master-Pixel-to-GeoModelXML, setting up python test integration scripts in order to do this. We will first test that something sensible shows up for the material using the LengthIntegrator tool.

P.S. This MR supersedes !37833 (closed) which had a very messy commit history. The relevant commits were cherry-picked to a clean branch. Apologies!

cc: @nstyles @asalzbur @hhayward @tlari