Add ultra-simple tracking manager integration

[sethrj]

Follow-on to #1606; see #1600. This defines static singleton classes used for integrating with external applications. It carefully checks usage patterns for single and MT use cases. Currently only the "tracking manager" version is implemented; a "user action" one will follow for older versions of Geant4.

[github-actions]

Test summary

 4 497 files   6 923 suites   13m 15s ⏱️
 1 698 tests  1 692 ✅  6 💤 0 ❌
24 285 runs  24 201 ✅ 84 💤 0 ❌

Results for commit 40cf8da.

♻️ This comment has been updated with latest results.

[amandalund]

Thanks for putting this together @sethrj!

[sethrj]

From @whokion at #1615 (comment):

One more minor comment on TrackingManagerConstructor: this class name is confusing as it sounds like constructing a TrackingManager rather than a PhysicsConstructor. A concrete class of G4VPhysicsConstructor is meant to be added to a modular physics list, so its name may be better to contain "Physics" (this is also a common practice in all Geant4 physics constructors). So, suggested name may be OffloadEmPhysics or something similar. Also, the OffloadParticles still be included in ConstructParticle() rather than empty even though they are overloaded somewhere (where?) without any side effect, which clarifies what particles are specifically involved for this physics constructor. On the other hand, this is still purely a semantic argument.