Detect Geant4 Multithreading Capabilities and Act On Them

This fixes #219 (closed)

If G4 lacks MT capabilities, we do not call allow_multithreading() in the GeometryBuilder, effectively switching it off for APSQ. We already properly instantiate a regular G4 RunManager and can still use APSQ in single-threaded mode.

Ref https://geant4-forum.web.cern.ch/t/definition-of-the-g4multithreaded-flag/3681/6

Errors are printed:

|13:57:25.682|   (ERROR) [C:GeometryBuilderGeant4] Geant4 has been built without multithreading support, forcing multithreading off.
                                                   To allow multithreading, rebuild Geant4 with the GEANT4_BUILD_MULTITHREADED option enabled.
|13:57:25.686|  (STATUS) Loaded 7 modules                     
|13:57:25.686|  (STATUS) Initializing 27 module instantiations
|13:57:25.893|  (STATUS) Initialized 27 module instantiations
|13:57:25.893|   (ERROR) Multithreading disabled since the current module configuration does not support it
|13:57:25.893|  (STATUS) Starting event loop