Michael Mayer / ctools

…odel with a standard diffuse cube background model that can be used for stacked analysis. The model container can be either retrieved using the models() method, or it can be written to disk with the save() method.

…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.

…ovided by the base class

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