Implement CPU-only G4MagneticField wrapper

[sethrj]

To facilitate integration with CMS (#1143) it should be straightforward to add a helper class to wrap the current Geant4 magnetic field .

1. When instantiating the local field inside a step, look up the geant4 logical volume's current region.
2. Get the field manager from the current logical volume
3. Convert the input coordinates to the field, convert the output from [T] to native units