Better access / visualization of grid search results

[amueller]

The GridSearchCV.grid_scores_ are stored in a not very user-friendly way.
It would be great to have nd-arrays and a set of labels.
Things that should be easily possible:

• plot in the case of 1 parameter
• matshow in the case 2 parameters
• look at slices and "marginals' (mean over rest of parameters) when looking at more parameters

[agramfort]

+10

[amueller]

This might be one of the issues I'll tackle next. But there are so many things I'm working on atm I'm task-switching way to much....

Thanks for your comments and reviews btw :)