RegressionModel fit statistics

This makes `RegressionModel` a little more like `MNLLocationChoiceModel` in that it has a `.fit_parameters` attribute with `'Coefficient'`, `'Std. Error'` and `'T-Score'` columns. It also has a `.report_fit()` method now. Both the `.fit_parameters` and `.model_fit` attributes of `RegressionModel` each have `.rsquared` and `.rsquared_adj` attributes and those are saved to YAML as `fit_rsquared` and `fit_rsquared_adj`, respectively.

After loading a `RegressionModel` from YAML it's `.model_fit` attribute now looks a little more like an actual statsmodels result with `.params`, `.bse`, `.pvalues`, `.rsquared` and `.rsquared_adj` attributes.

The `coefficients` field is gone from the saved YAML and replaced by `fit_parameters`.