Iftach Sadeh / ctools

So far any exception occuring during the construction of the ctools were not catched. This is corrected now.

Added ctool::require_inobs_nocube() method that throws an exception if a counts cube has been specified as inobs parameter. This method is then used by ctbin, ctexpcube, ctpsfcube and ctbkgcube.

… GCTAExposure to GCTACubeExposure and GCTAMeanPsf to GCTACubePsf).

…(see #1360 for details).

…)-function of individual ctools

…d scave() methods to ctool interface. Add read_ahead() method to handle reading of task parameters at the beginning of the task execution. Remove all m_read_ahead members from the derived classes.