Change ctmodel logic in a way similar to ctbin.

ctmodel now always produces a single model cube on output. There are two important input parameters: "infile" and "obsfile". "infile" specifies a counts or model cube that is used to define the binning of the model cube. The functionnality for this parameter is as before. If "infile=NONE", the cube will be defined according to task parameters that specify the spatial and the energy binning. The "obsfile" parameter is new and allows specifying the observation definition for which the model cube should be computed. If an event list or a counts cube is presented, the pointing and Good Time Intervals will be extracted from the FITS file header. If an observation definition XML file is presented, ctmodel will now loop over all CTA observations in the file and add the model values to the model cube. If no response information is associated to the "obsfile" (which is typically the case for event lists or counts cubes), the "caldb" and "irf" parameters will be used to identify the response.
Compute model values only for bins that are within the energy boundaries of the observation.
Add support folder and ctool base class
Use ctool base class in ctbin tool.

The ctool base class is now used in the ctbin tool. The get_ebounds() method has been moved to the ctool base class so that it can be used also in other tools. This reduces code duplication.
ctbkgcube now derives from ctool class, removed get_ebounds() method as it is provided by the base class
ctexpcube now derives from ctool. Added set_obs_response() to the ctool base class for CTA observation response setting.
ctlike now derives from ctool base class.
Merge branch '1330-ctmodel-multiple-handling' into add-base-class
ctmodel now derives from ctool. Add ctool::set_response() method that sets the response for all CTA observation in the container that have not yet a response using the task parameters.
ctobssim now derives from ctool base class.
ctpsfcube now derives from ctool base class.
ctselect now derives from ctool base class.
ctskymap now derives from ctool base class.
cttsmap now derives from ctool base class.
ctcubemask now derives from ctool base class.
Add information about ctool base class.
Move execute() method to ctool base class and add pure virtual clear(), run() and 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.