[WIP] Ridge path

Adressing https://github.com/scikit-learn/scikit-learn/issues/3373

A first implementation of a ridge path using `'eigen'` solver. This solver is now available in `ridge_regression`, which calls upon a reduced functionality of `ridge_path`.

Next steps:

- The same path approach can be straightforwardly used with SVD as well: Write svd path and replace in ridge_regression
- Attack `RidgeCV`: Replace the `GridSearchCV` by `ridge_path` if `eigen` or `svd` are selected.
- Incorporate `ridge_path_looe` from the gist https://gist.github.com/eickenberg/e9eb106f12e40d017fd0
- Adjust `RidgeCV` to use the looe functionalities


Added new logic for `RidgeCV` in the case of `eigen` and `svd` solvers. Still falling back to `GridSearchCV` for the others. Probably the best ideas is, as discussed, only to allow solvers that actually provide a speedup with respect to just wrapping using `GridSearchCV`.