Add load() and save() methods to GCTAObservation.

GCTAObservation::load() methods have been added that automatically recognize whether the file is an event list or a event cube. This makes the load_unbinned() and load_binned() methods obsolete. A variant has also been added that loads data directly from a FITS object.

A GCTAObservation::save() method has been implemented that saves data directly into a FITS object, without storing that object necessarily on disk.