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.
then we should change `fit_sphere_to_headshape` to look for points of type CARDINAL, at the moment it looks for points of type EXTRA. There is probably a reason for this: maybe some devices export the head digitization points as EXTRA?