Add support for general GModelData simulations.

Background simulations can now be done for any model of type GModelData. This is thanks to the abstract mc() method that has been introduced in GModelData. A test case for the new GCTABackground model has been added to the unit tests.
Fix all parameters for the moment.
Add some unit tests
Fit prefactor and index.
Removed debugging sleep instruction
Adapt to new GCTARoi and GCTAInstDir interface.

The GCTARoi::contains() method is now used to check for ROI containment.
Add testing of diffuse map cubes to Python unit test.