Reformat source code to make it compliant with coding conventions.
Adjust copyright and header text.
Reformat source code to make it compliant with GammaLib coding conventions.
Add pure virtual mc() method to GModelData class.

All models derived from GModelData now need to implement a mc() method. The abstract mc() method returns a pointer to GEvents*, thus allowing a method to either simulate an event list or an event cube.
Evaluate function and error also if there were no parameters to fit.

Before, no evaluation was done if there was no parameter to fit, giving an empty model in case that all parameters were fixed. Now, the final evaluation is done in all cases.