GammaLib

Some bugs have been found in relation with the following new GCTAModelBackground constructor:
GCTAModelBackground::GCTAModelBackground(const GCTAObservation& obs,
const std::string& filename,
const GModelSpectral& spectral,
const int& nx_sky,
const int& ny_sky,
const int& n_energy)

and the related function

void GCTAModelBackground::set_spatial(const GCTAObservation& obs,
const std::string& filename,
const int& nx_sky,
const int& ny_sky,
const int& n_energy)

The needed fix are:
- in the constructor: initialisation of m_temporal
- in set_spatial: correct filling of the GEnergies in the case m_energy=0
- in @set_spatial: add GException in case nx_sky or ny_sky or n_energy is <0 (not really a bug but taken care of here anyway)