ENH: Add interpolation code and example

cc / FYI @agramfort @wmvanvliet @MadsJensen @t3on @christianbrodbeck 
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> I agree that *this* code is one line, but the problem is that to do this properly requires a few lines -- you have to check to make sure none of your norms are zero to avoid corner cases with warnings.
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+0.5 then :)