Thierry Reposeur / 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.

* Checked the code for coding conventions.
* Renamed the angular seperation user parameters to amax and anumbins. "a" stands for angular separation, "d" for delta is less explicit.
* Add ctpsfcube executable unit test