Olga Sergijenko / gammalib

…ients argument (#1847)

The eval_gradients() methods duplicate much of the code of the eval() methods. To increase the maintainability of the code, the eval_gradients() methods have been removed, and the computation of partial derivatives is now done by the eval() methods if the optional gradients argument is set to true.

… including docs and unit tests

  - New naming scheme.
  - New utility functions/constructor supporting cutoff-energy.
  - Usage of new helper functions for xml read-in/write-out.

The GModelSpectralRegistry::alloc() method now also checks for the existence of model parameters and allocates the appropriate model from the registry that is found in the XML file base on model type and parameter names. This allows now to have the same model type but different parameter names in the registry.

The spectral model classes have been adapted to the new logic, and models have been added to the registry for all parameter combinations that may exist.

The unit tests and documentation have been updated.

The following new names have been introduced for the spatial model components:
* "PointSource" (alias: "SkyDirFunction")
* "RadialDisk" (alias: "DiskFunction")
* "RadialGaussian" (alias: "GaussFunction")
* "RadialShell" (alias: "ShellFunction")
* "EllipticalDisk"
* "EllipticalGaussian" (alias: "EllipticalGauss")
* "DiffuseIsotropic" (alias: "ConstantValue")
* "DiffuseMap" (alias: "SpatialMap")
* "DiffuseMapCube" (alias: "MapCubeFunction")

Also document the formulae in the method description.