ENH: Add interpolation code and example

cc / FYI @agramfort @wmvanvliet @MadsJensen @t3on @christianbrodbeck 
...and something like `assert_allclose` would be a lot more informative here, since you don't have to think about what the units are for EEG. Are the interpolations not that good actually since the data is on the order of 1e-5 to 1e-6 anyway?