Remove arguments from GRan::normal() method.

The methods in GRan should be as simple as possible, and for drawing normal random numbers no arguments are needed as the values can be scaled later. The arguments have thus been removed.
Add dummy response function.
Remove gauss68containment2sigma constant.

The constant is so far only used in a single place, and the name is also not very clear, and much too long.
Change code formating
Finalize implementation of energy dispersion for CTA.

The GCTAResponse::nedisp method has been implemented and tested. A corresponding unit test has been added to the CTA Python test script test_CTA.py.

To support this method, GCTAResponse::ebounds_src and GCTAEdispPerfTable::ebounds_src and GCTAEdispPerfTable::ebounds_obs methods have been implemented. The ebounds_src method returns the true energy boundaries of the energy dispersion for a given observed event energy. The boundaries are defined by 5 times the Gaussian sigma value. The ebounds_obs method returns the observed event energy boundaries of the energy dispersion for a given true photon energy. Again, a boundary of 5 times the Gaussian sigma value is used.