Abbreviations in attribute names

[jtrakk]

I don't always remember what the abbreviations stand for and I have to go back to the docs frequently. It would be easier to understand if .mf were replaced with .modelframe, for example.

[palday]

I think part of the motivation here is that the public API is largely defined by methods and not fields. For example model_response, modelcols and modelmatrix provide accessors for those internal fields.

(There are some members of the community who think that the API should be exclusively defined by methods and the type hierarchy and that the details of any given struct should always be considered private implementation details.)