ENH: Add interpolation code and example

cc / FYI @agramfort @wmvanvliet @MadsJensen @t3on @christianbrodbeck 
It's the same order I think it depends on the sensor config and the signal redundancy

> On 12 Dec 2014, at 18:53, 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)
> ...and something like assert_allclose would be a lot more informative here, since you don't have to think about what the units are for EEG. Are the interpolations not that good actually since the data is on the order of 1e-5 to 1e-6 anyway?
> 
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