Remove methods duplicated from GSkyDir and add reference access to GSkyDir member.

In order to reduce code duplication, all methods that were copied from GSkyDir have been removed and direct access is now provided to the GSkyDir member for instrument direction handling.
Add contains() method.

The contains() method determines whether an event is contained within the ROI.
Add pure virtual contains() method.

Every ROI class now needs to implement a contains() method that checks whether an event is contained within the ROI. This should enhance the abstract handling of regions of interests.
Add contains() method.
Add contains() method.
Do not remove conversion constructor as it is still needed in some scripts.