Begins to implement gradients in GModelSpectralGauss::eval_gradients
Begins to implement gradients in GModelSpectralGauss::eval_gradients
Implements eval_gradients in GModelSpectralGauss - not yet checked
Corrects the mc loop in GModelSpectralGauss.cpp, in GModelSpectralGauss::mc, as suggested
Corrects the mc loop in GModelSpectralGauss.cpp, in GModelSpectralGauss::mc, as suggested
Implements GModelSpectralGauss eval_gradients method and changes the GModelSpectralGauss mc method as suggested
Merge branch 'issue_1014_gradients' into integration
Correct mc() and eval_gradients() methods and rename "Prefactor" to "Normalization".

The mc() method went in endless loops as the exit condition of the while loop was inversed (the method continued looping when the energy was within the valid range and only exited when it was outside).

The eval_gradients() method computed the gradients with respect to the parameter values and not the parameter factors. All gradients have be multiplied by the scales. Also, the gradients are now set to 0 if the parameters are fixed.

Finally, the parameter "Prefactor" was renamed to "Normalization" because the parameter gives the integral flux under the Gaussian, not the prefactor.