Rework CTA ROI and instrument direction classes.

A contains() method has been added to GCTARoi that checks whether an event in contained within the ROI. This method will become pure virtual in the GRoi class later so that the check can be done independently of the ROI specifications.

The contains() method is now used in GCTAModelBackground for checking whether an event lies within the ROI.

The GCTAInstDir class now only has the dir() method to access and modify the GSkyDir member. All methods that were basically copied from GSkyDir have now been removed to avoid code duplication. Using the dir() method, all methods are now directly accessible from the GSkyDir object.