ENH: Add interpolation code and example

cc / FYI @agramfort @wmvanvliet @MadsJensen @t3on @christianbrodbeck 
...in other words, this function seems to have nothing having to do with a head in it currently, so it should probably be renamed, and it would be more convenient import-wise to have it be in `mne.channels` somewhere. The headshape-fitting function that uses this should probably also be moved over there, since it's more a property of the channels / info than being maxfilter-specific.