Athena code migrated from in-house GeoModelKernel to standalone GeoModelKernel library

Given the standalone library is based on Eigen instead of CLHEP, all GeoModel clients
in Athena code were also migrated to the Eigen-based interface of GeoModel. This affected
a large number of packages. Most significant changes have been introduced to subsystem
GeoModel packages.

The in-house GeoModelKernel code has been removed from the repository.

Almost all header files in PixelGeoModel have been moved to the src directory.
The next step should be turning this package into a component-only library.
Similar code reorganization can be applied to SCT_GeoModel too.

Subsystem readout geometry interfaces remain mostly unchanged, except for a minor
change in the InDet readout geometry interface (which was unavoidable).

Several non-GeoModel packages affected by this migration mostly required addition
of missing CLHEP includes.