Renamde GCTAModelBackground to GCTAModelCubeBackground.
Rename GCTAInstBackground to GCTAIrfBackground and rename GCTABackground to GCTACubeBackground
Rename background model XML files.

Files names are now more consistent with the actual background model.
Rename GCTAModelInstBackground to GCTAModelIrfBackground
Rename GCTAModelBackground to GCTAModelCubeBackground and GCTAModelInstBackground to GCTAModelIrfBackground
Rename GCTAModelInstBackground to GCTAModelIrfBackground