Add unit set operator.

The unit set operator allows setting of energies using a syntax like:

  GEnergy eng;
  eng(1.0, "TeV");
Implement energy unit management in read and write methods.

When energy boundaries are read from a FITS file, the energy units are set according to the FITS header keyword. If no keyword is provided, units are set to keV.

Upon writing, the energy units can be specified as an argument.
Add etrue() and emeasured() methods that return the boundaries for a specific energy.
Add more tests for GRmf class.
Review code and implement new Monte Carlo cache. Correct mc() method.
Add Python interface for GCTANackgroundPerfTable.
Complete Python interface.
Correct GCTAEdispRmf::set_mc_cache method and add filename to GCTAEdispRmf::print method
Add Python unit test for CTA response classes (embryon)
Automatically load GCTAEdispRmf in GCTAResponse