move model-specific conversion code into the model files

[pkienzle]

The current scheme for updating models between releases relies on central conversion table in sasmodels/conversion_table.py as well as model specific code in sasmodels/convert.py.

Going forward, it would be better to encode the conversion in the model file, which would allow models shared on the marketplace to be updated while preserving backward compatibility with previously saved fits.

Changing the model name will require a different mechanism. When loading the saved project, need a way to identify the new model name given the model name in the file. It is even more complicated if models are combined such as rectangular prism and parallelepiped which are the same model just with different parameterization.