Use observed energy as default for background access operator.
Add GCTABackgroundPerfTable class.

The GCTABackgroundPerfTable class models an instrumental CTA background model from a performance table file. Interpolation in energy is done logarithmically.
Implement logarithmic energy interpolation in GCTABackground3D.
Replace exp() by std::exp().
Simplify code.
Replace exp() by std::exp()
Implement new instdir() method.

The new instdir() method makes use of existing GSkyDir methods which benefit from value caching, and thus potentially are faster.
Reformat code.