Switch custom regressor for sklearn

[FNTwin]

From: #45 (comment)

Currently the linear regression and ridge regression (+ parameter confidence) are coded from scratch in a separate and stand alone module. We can provide/change them with a more robust API like sklearn.
The only issue (not even a big issue) is introducing a new and more complex dependency like sklearn and maybe rewrite a bit the logic for extracting the confidence interval from the parameters.