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Declare events() access method as virtual.
Load events only when needed in the GCTAObservation class.

The events are now only loaded when needed, reducing the memory usage for CTA data. The method dispose_events() has been added which explicitely allows the disposal of the events.
Move read_ds_roi() method from GCTAEventList class into GCTASupport module.

The method has been moved as it is now also used by the GCTAObservation class.
Add roi() method.

The roi() method returns the Region of Interest of an event list even if no event list is actually loaded in the CTA observation.
Avoid using the event list in fill() methods.