Add documentation to Custom Fit model generated by GCC

[butlerpd]

The current doc string is not helpful. In particular understanding the two new parameter (volume and swelling) and how to use the model would be very helpful. In fact it may be that some of the documentation in the rst should be moved here? Since the new documentation editor feature now will render the docs for plugins.

[wpotrzebowski]

Is it still the issue?

[butlerpd]

Yes - but minor issue. This is about creating model documentation not writing regular documentation. In other words part of the model.py file. The suggestion is that the template that creates the model from a Debye calculation off of a PDF file could include the usual model documentation at the beginning with some information for the user.

However I would rate this as a low priority compared to everything else.