ENH: Add interpolation code and example

cc / FYI @agramfort @wmvanvliet @MadsJensen @t3on @christianbrodbeck 
Not worth it, it's super fast

> On 12 Dec 2014, at 18:40, Eric Larson <notifications@github.com> wrote:
> 
> In mne/channels/interpolation.py:
> 
> > @@ -0,0 +1,162 @@
> > +import numpy as np
> > +from numpy.polynomial.legendre import legval
> If you want your code to be faster, I use tables for Legendre values for forward computation. You might be able to use them here, too, not sure.
> 
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