Shifted all of the dispersion model descriptions and parameter lists to the variables module.
The descriptions and parameter lists which were part of the relax_disp.select_model user function
backend have been shifted into the specific_analyses.relax_disp.variables module as MODEL_DESC_* and
MODEL_PARAMS_* variables. The descriptions have also all been standardised.
The MODEL_DESC and MODEL_PARAMS dictionaries have also been created to hold all of the descriptions
and parameters in one place.
