Implemented GUI commands.

Commands added:
- StepMax: sets the maximum stepping distance within the
  sheilding/detector.
- SheildingThickness: sets the thickness of the sheilding (requires
  UpdateGeometry to be run afterwards).
- SheildingMaterial: sets the sheilding material.
- UpdateGeometry: forces the geometry to be completely updated.
- SetHitsFile: filename where the outputs are saved
- He3Abundance: sets the amount of He3 in the detector.

Signed-off-by: JP-Ellis <coujellis@gmail.com>