Allow simulation to run Geant4 macros/UI

[drbenmorgan]

As described in #193, we may be seeing overlaps in the simulation geometry. This motivates allowing the simulation to either:

• run a Geant4 macro
• start simulation in interactive (Geant4 UI shell) mode

These are primarily for debugging purposes so only needs a limited interface. The canonical use case is to check for geometry overlaps as described here

Partially helped by #191, but that won't implement this feature, just provide a smoother path to enable it.