GammaLib

A GCTAObservation filename constructor should be added to allow loading of a file upon construction.

For the moment, the GCTAObservation(const std::string& instrument) constructor is used for the registry. The constructor should be modified, for example by adding a dummy argument

GCTAObservation(const bool& dummy, const std::string& instrument)

so that the registry globals can be modified to

const GCTAObservation      g_obs_cta_seed("CTA");
const GCTAObservation      g_obs_hess_seed("HESS");
const GCTAObservation      g_obs_magic_seed("MAGIC");
const GCTAObservation      g_obs_veritas_seed("VERITAS");

and we have freedom to build a filename constructor.