Implement still simplified GCaldb::filename method.

The GCaldb::filename method returns an absolute IRF filename based on a set of selection criteria. The selection criteria are still very simple, and a more sophisticated should be implemented in the futur. For the time being, the method does the job.
Add dummy IRF.
Remove old Prod1 performance tables from repository.
Remove old IRF files
Add new Prod1 calibration files (for testing purposes)
Update calibration index file after updating IRF test files.
Install caldb instrument response functions
Adapt unit test to new PSFs and add GCTAPsfKing class tests.
Adapt unit tests to new IRF structure.
Reconsider calibration database handling.

Instead of passing a string providing the calibration database path, the GCTAObservation and GCTAResponse classes now take a GCaldb object. This allows for a more flexible handling and selection of response functions through passing the corresponding calibration database object.

The rmffile member and method has become obsolete in GCTAResponse, so it was deleted. Now everything is handled by the GCTAEdisp class.
Add gammalib::dir_exists() function.
GCaldb::filename selection method is now case insensitive.

The method converts the selection strings to upper case before comparison.