GFunction::eval method now takes a const double precision reference as argument.

The GFunction::eval interface has been revised and all affected GammaLib classes have been adapted.
Revise interfaces.
GNodeArray now derives from GContainer class.

The node array class now derives from the container base class. All typical container methods, such as at(), size(), isempty(), append(), insert(), remove(), reserve() and extend() are now implemented. Unit tests have been added to check the functionnality.

The Python interface now implements the __len__ method on the GContainer class level. This allows to determine container sizes using the len() function. The __len__ method is now obsolete in the derived classes and thus has been removed.
Review GTools and GCsv class.

Some arguments that were not already passed as const references are now declared as const references. Inline methods were added to GCvs.
Put methods inline.
Catch pixel-to-spherical transformation exceptions in GCTAEventCube::set_directions.

When using the Hammer-Aitoff projection, the pixel-to-spherical used in GCTAEventCube::set_directions may fell as pixels may correspond to zones that do not represent sky regions. To prevent this problem, any exception is catched and the solid angle of the pixel is set to 0. This has the nice side effect of filtering out the bin during model fitting as the solid angle is used to multiply the model value.