Put constants into gammalib namespace.

All constants are now defined within the gammalib namespace.
Adapt to constants that are now defined within the gammalib namespace.
Put functions in gammalib namespace.
Adapt to GTools functions that are now put in gammalib namespace.
Use native toupper and tolower functions in gammalib::tolower and gammalib::toupper functions.
Correct formatting in print() method.

During the rework of the print() method following the introduction of the gammalib namespace, some extra linefeeds have been introduced that have now been removed again.
Add char *__str__() extension to implement the print() method.
Use gammalib namespace to call tochar method.
Remove const par() method as swig ignores it anyway.
Remove const at() method and __str()__() extension.

The const at() method is ignored by swig anyway, as Python has no const declaration.

The __str()__() extension is now implemented in the GBase base class, so we don't need to implement it again for the derived class.
Remove __str__() extension as it is now implemented in the base class.
Use gammalib namespace for GammaLib functions.
Put cta_roi_arclength() in gammalib namespace.

The cta_roi_arclength() is now defined in the gammalib namespace.
Rename from GNumerics.hpp to GMath.hpp
Rename from GNumerics.cpp to GMath.cpp
Use GMath.hpp instead of GNumerics.hpp.

GNumerics.hpp has been remaned to GMath.hpp.
Declare function in gammalib namespace.

All functions are from now on defined in the gammalib namespace.
Use gammalib namespace for gammln function.
Move arccos and modulo function from GTools to GMath.

As arccos() and modulo() are mathematical functions they are better placed in GMath.
Add #include "GMath.hpp" as arccos() and modulo() are now in GMath module.