Revert "GModels access operator now returns reference instead of pointer."

This reverts commit 813252eb589e317fd139d0cdde4f20bdd7959ef2.
Add set() method and operator[] returns pointer instead of reference.

To prevent code slicing and to conform to the GammaLib standard, the operator[] of a pointer container should return pointers and not reference. The interface has thus been changed. To allow also setting of elements, the set() method has been added.
Remove checks of pointer validity.

Checks for pointer validity are not needed, as pointers should be always valid by construction.
Modified GModels and GObservations interface.
Merge branch '692-interface-review' into integration

Conflicts:
	src/model/GModels.cpp