Add abstract base class GCTABackground.

The GCTABackground class is the abstract base class for all CTA background models. It allows handling of background models at the same level as the instrument response functions.
Implement background model support in GCTAResponse and GCTAObservation.

The CTA background model, represented by the abstract base class GCTABackground, has been added as 4th component of the instrument response. The CTA observation definition file has been updated by adding a "Background" parameter to the observation tag, allowing the specification of a filename for the background model.
Add GCTABackground3D and GCTAInstBackground classes.

The GCTABackground3D class implements the handling of 3D response tables containing background information in the instrument system. The three axes of the table are DETX, DETY and ENERG.

The GCTAInstBackground class implements a background model that will use the instrumental CTA background. It derives from GModelData.
Rename GCTAInstBackground to GCTAModelInstBackground.

To make clear that GCTAInstBackground is a model it has been renamed to GCTAModelInstBackground, in agreement with the other model components in the CTA module.
Add unit test for GCTAModelInstBackground.

A unit test has been added for GCTAModelInstBackground that makes use of an example XML file for testing the constructors and the XML read() and write() methods.
Add element access operators.

The element access operators allow accessing response table elements by element index. The single parameter variant returns the element of the first table, the two parameters variant allows to specify the table index.
Add nodes() method that returns the axis nodes.
Add log10 and log10erg methods.

The log10() method allows specifying the energy unit in setting the energy value. The log10erg() methods allow setting and retrieving and energy in log10 of ergs.
Correct coordinate transformation in GCTAPointing::skydir method.

The coordinate transformation was not correct before. Not it should do what it's expected to do.
Add detx and dety output to GCTAInstDir::print method.
Add GEnergy::operator() that returns energy as function of specified unit.

The GEnergy::operator() allows to specify the energy unit as a string, and returns the energy value according to the string as double precision value.
Remove m_detx and m_dety from GCTAEventAtom and use GCTAInstDir members instead.
Implement Monte Carlo sampling for CTA instrumental background model.
Add Python bindings for GCTABAckground3D and GCTAModelInstBackground classes.
Implement eval() and eval_gradients() methods in GCTAModelInstBackground.

The eval() and eval_gradients() methods are now implemented in GCTAModelInstBackground. An interpolation operator has been added to the GCTABackground interface and implemented in GCTABackground3D to provide access to the energy dependent background rate.
Implement GCTAModelInstBackground::npred method.
Use observed energy as default for background access operator.
Add GCTABackgroundPerfTable class.

The GCTABackgroundPerfTable class models an instrumental CTA background model from a performance table file. Interpolation in energy is done logarithmically.
Implement logarithmic energy interpolation in GCTABackground3D.
Replace exp() by std::exp().