Wrijupan Bhattacharyya / ctools

… GCTAExposure to GCTACubeExposure and GCTAMeanPsf to GCTACubePsf).

…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.

…ss for CTA observation response setting.

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