GammaLib

All CTA background models should be put under the umbrella of the GCTAModelBackground class.

Specifically, GCTAModelAeffBackground, GCTAModelCubeBackground, and GCTAModelIrfBackground should be derived from GCTAModelSpatial as they are just specific spatial components of the general CTA background model.

Also, GCTAModelRadialAcceptance should be removed, and it remains to be seen whether we really should keep specific radial classes, such as GCTAModelRadialGauss, GCTAModelRadialPolynom, GCTAModelRadialProfile, or whether we should not simply put them under the GCTAModelSpatial umbrella.