Sarka Todorova correctly noted that the public loaders should return
a const 'world' volume, so a const pointer to the GeoVPhysVol.

I added constness to both the GeoModelRead::buildGeoModel() and to the
new loadDB() helper methods.

I then remove the constness in the visualization code, internally,
otherwise it's more complicated to manipulate the geo tree.

This reverts commit 4484186b.

Following the inetrface change in the GeoModelIO Write and Read, where I replaced the GeoPhysVol with GeoVPhysVol, now any GeoPhysVol or GeoFullPhysVol can be used as the Root/World volume.

All GeoModel targets provide ALIAS targets to their namespaced
equivalents as exported for use later as imported targets. This
renders obsolete the need to check for full/local builds and use different
target names.

Remove use of .._INDIVIDUAL_BUILD variables, consistently link to
namespaced targets.

TODO: fix this in FullSimLight due to incomplete use of FullSimLight
interface target.

- Make builtin "imported" target depend on JSONExt external project.
  - Subprojects only then need to link to the nlohmann_json target
    whether builtin/external is used
- Use REQUIRED to find system nlohmann_json as modern installs will
  always provide a CMake config script.

Here is an additional tweak to a few programs.  These are designed to obviate the need for users to specify environment variables pointing to certain directories which hold the plugins and configuration files

some customizations for ATLAS muon system.  While this is special to ATLAS, we dont think it a bad idea to keep these customizations here because they serve as an example others may wish to follow