Changes

Test offload examples in each Geant4 threading model

For each offload example, add a test that runs it with each of Geant4's
threading types:

- Serial
- MT (standard C++ threads)
- Tasking (PTL-based tasks+threadpool)

Only enable the latter two if the found Geant4 supports them.

Clarify implementations of physics table functions

- Use contracts on pointers that require deref
- Follow GeantImporter implementation for loop over processes

A cleaner range-based for over processes isn't possible due to
G4ProcessVector not providing a directly compatible interface.