WIP better Pandas support.

Don't convert dataframes in grid search and cross-validation.

If we can agree on this, I'll
* [ ] add tests
* [ ] rename allow_lists to force_array (and invert the meaning). It's an internal parameter anyhow. 
In the mean time we can just add a comment next to this line to explain that `allow_lists` is used as a pass through flag that is also useful for non-list data-structures in model evaluation and selection tools.