Cleaned up `CMakeLists.txt` files in subdirectories so that the minimum reqiured
version is set only at the beginning of the top-level `CMakeLists.txt` file

Modified:
* GeoModelExamples/HelloGeoRead/CMakeLists.txt
* GeoModelExamples/HelloGeoReadNodeAction/main.cxx
* GeoModelExamples/HelloGeoWriteReadWrite/main.cpp
* GeoModelTools/GMCAT/src/gmcat.cxx
* FullSimLight/include/MassCalculator.hh
* FullSimLight/src/FSLDetectorConstruction.cc
* FullSimLight/src/MassCalculator.cc

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.

- Logically organise top level script
- Compile implementation into OBJECT library to which each executable
  links. Reduces recompilation of same files for each executable.
- Use target properties to transmit usage requirements

Plugins currently rely on an include directory to use fullsimlight's
plugin interface, but it already provides a FullSimLight target for this.

Set required INTERFACE properties for FullSimLight target and add a
namespaced ALIAS target for it. Update Plugin builds to use this target.

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 Eigen external project.
- Subprojects only then need to link to the Eigen3::Eigen target
  whether builtin/external is used

- Make builtin "imported" target depend on XercesCBuiltIn external project.
- Subprojects only then need to link to the XercesC::XercesC target
  whether builtin/external is used

- 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.

TODO: NEEDS UPDATE AFTER REBASE

Use std::unique_ptr for static member pointer.  And, stop checking if an unsigned integer is greater than or equal to zero. Both were flagged in warning messages which are now silenced, but I believe the first issue could have led to a crash in the right circumstances

This is a bugfix:  the pythia dependency of fullSimLight is declared before the target (fullSimLight) has been delclared