Olga Sergijenko / gammalib

# Conflicts:
#	src/obs/GObservation.cpp
#	src/obs/GResponse.cpp

A major bug was lurking in the GModelSpatialDiffuse, GModelSpatialElliptical and GModelSpatialRadial classes where the GModelSpatial base class members were not copied in the copy constructors. This prevented in particular the model fitting of diffuse maps in the stacked analysis.

…ack for model parameters with analytical gradients at the level of the GObservation class.

…ssian models and stacked analysis (#3203)

…dependent() methods (#3203)

Implemented the following methods:
- GVector GModel::eval(GObservation&, GMatrixSparse*)
- GVector GModelSky::eval(GObservation&, GMatrixSparse*)
- GVector GResponse::convolve(GModelSky&, GObservation&, GMatrixSparse*)
- GVector GObservation::model(GModels&, GMatrixSparse*)
- GVector GObservation::model_grad(GModel&, GModelPar&)

Bisection is substantially faster then the previous looping over all elements in a column.