Add Python bindings for GCTABAckground3D and GCTAModelInstBackground classes.
Implement eval() and eval_gradients() methods in GCTAModelInstBackground.

The eval() and eval_gradients() methods are now implemented in GCTAModelInstBackground. An interpolation operator has been added to the GCTABackground interface and implemented in GCTABackground3D to provide access to the energy dependent background rate.
Implement GCTAModelInstBackground::npred method.