Remove duplication of _Fit.params and _Fit.parameters

[pgkirsch]

params is array, parameters is a dictionary, both have some value but the similar names are confusing.

[whoburg]

believe the parameters name came from Levenberg–Marquardt literature

[pgkirsch]

I figured there was a good precedent for it. My issue isn't with our use of parameters, rather that I clumsily introduced a new attribute parameters while we still also have params (the variable name that has been around since OG GPfit). Lazy on my part and needs to be deconflicted, just not sure how best to do it yet.

[whoburg]

gotcha, sounds good.