ENH: Add interpolation code and example

cc / FYI @agramfort @wmvanvliet @MadsJensen @t3on @christianbrodbeck 
If you want to be more efficient and try a fun challenge (at least I find it challenging and fun), you can probably accomplish all this with `np.einsum` instead of `np.dot` followed by `np.transpose`. But if you do it, please leave the original code there, commented out, to make the `einsum` operation more explicit :)