WIP: Statsmodels chi2 fitting

This is an example notebook for the fixed scale covariance type @josef-pkt  is adding in #2137.

It's work in progress ... I get an error from `WLS` and the `curve_fit` results don't match the reference results from the paper.

Also ... is it already possible to fit non-linear models with this covariance type?
If not, maybe this can be added and then I include an example in this notebook?