Adapt to new GCaldb interface.

A new logic has been implemented concerning the caldb parameter. If the name specified by caldb is a directory, the directory is set as the root directory of the calibration database. The file specified by irf must then be a file provided within this root directory.

If the name is not a directory it is used as the mission name that is used to build the calibration directory path. The irf is then the response name.
Calibration database parameters are still automatic.

We don't want to hide these parameters but parse for the instrument.