ENH: Add interpolation code and example

cc / FYI @agramfort @wmvanvliet @MadsJensen @t3on @christianbrodbeck 
:) :) hehe

> On 12 Dec 2014, at 18:52, Eric Larson <notifications@github.com> wrote:
> 
> In mne/channels/tests/test_interpolation.py:
> 
> > +    goods_idx = np.ones(len(epochs.ch_names), dtype=bool)
> > +    goods_idx[epochs.ch_names.index('EEG 012')] = False
> > +    bads_idx = ~goods_idx
> > +
> > +    evoked = epochs.average()
> > +    ave_before = evoked.data[bads_idx]
> > +
> > +    pos = epochs.get_channel_positions()
> > +    pos_good = pos[goods_idx]
> > +    pos_bad = pos[bads_idx]
> > +    interpolation = make_interpolation_matrix(pos_good, pos_bad)
> > +    assert_equal(interpolation.shape, (1, len(epochs.ch_names) - 1))
> > +
> > +    ave_after = np.dot(interpolation, evoked.data[goods_idx])
> > +
> > +    assert_array_almost_equal(ave_before, ave_after, decimal=5)
> I don't know whether to be impressed by the quality of the interpolation or sad about EEG signal redundancy :)
> 
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