Parameter names in rheological models are slightly inconsistent

[akabla]

There are a few issues we can try to address in future versions.

• We use G for the generalised and empirical model, but k elsewhere.

• The subscript alpha does not exist in unicode, so we have a naming that is not very intuitive, with a and c_a for the first springpot, but \beta and c_\beta for the other one. I think we should avoid using subscripts with greek letters. They are hard to read and hard to type.

We now have a translation table for model symbols, so we can accepts many different syntax and avoid breaking changes. I just think that the default should be systematic, as much as possible.

[moustachio-belvedere]

May be of interest, but it's a long way out stevengj/subsuper-proposal#10