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
Check for model validity in GModels::eval.

Fixes bug #1153.
Correctly declare grad argument as const reference in edisp_kern constructor.

The grad argument was stored as a reference but declared by value. This led to the clang warning: binding reference member 'm_grad' to stack allocated parameter 'grad'.
Add parentheses around logical expression as required by clang.
Added -DSWIG_PYTHON_LEGACY_BOOL to the swig command for swig 3.0 compatibility problems with Booleans.