Fix EEG electrode positions in EDF/BDF import.

When importing EDF/BDF files, an optional .htps file can be supplied to
provide electrode coordinates. This commit fixes the way these are
stored in the .info dictionary, so that functions such as
make_eeg_layout can use them.
fit_sphere_to_headshape expects the points to be of kind EXTRA:

    # get head digization points, excluding some frontal points (nose etc.)
    hsp = [p['r'] for p in info['dig'] if p['kind'] == FIFF.FIFFV_POINT_EXTRA
           and not (p['r'][2] < 0 and p['r'][1] > 0)]

Whether or not this makes any sense I don't know.