Johan Bregeon / ctools

… 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.

…esponse for all CTA observation in the container that have not yet a response using the task parameters.

…ss for CTA observation response setting.

…bservation::hasresponse()

The ctbkgcube tool generates a background model for cube-style analysis.

The ctexpcube tool is now functional, a unit test has been added.