ENH: Add interpolation code and example

cc / FYI @agramfort @wmvanvliet @MadsJensen @t3on @christianbrodbeck 
> Yes. Let's make it EEG specific. We would nee to compute an interpolation matrix based on the code used in fieldlines

yes that would be the good way to do it !

maybe we could nerd snip @Eric89GXL with this ? :)