add class GModelSpectralSuperExpPlaw (header,source and .i-file)
Added example model and fixed minor calculation bug
Merge remote-tracking branch 'origin/1147-implement-GModelSpectralSuperExpPlaw' into integration
Review code for integration and add a unit test.

The GModelSpectralSuperExpPlaw code has been reviewed for integration into the development tree. Most changes were related to code formatting, some changes were needed in the XML interface to correctly reflect the model type "PLSuperExpCutoff".

A unit test method has been added to test_GModel, modelled on unit tests for other spectral models.