Topo plots for EEG data, even when no digitization points are present

Many EEG-only labs (including mine) don't bother with source localization and so don't mark EEG electrode locations. When plotting, relying on the approximate electrode positions reported by the cap manufacturer is good enough. In practice, this means ch['eeg_loc'] and ch['loc'] are specified, but no digitization points are present in the data. This breaks `make_eeg_layout`, which is currently very picky about which digitization points should be present. This PR makes the function a lot less picky:

 - Add flag to select which digitization points to use in
   `fit_sphere_to_headshape`. This defaults to only `FIFF.FIFFV_POINT_CARDINAL`, which was the default
   before I started messing with it in #1541. When calling the function from `make_eeg_layout`, more points
   are considered. I've noticed that Neuromag data adds `FIFF.FIFFV_POINT_EEG`, while the EDF loader adds
   `FIFF.FIFFV_POINT_EXTRA`, so it looks at both in addition to `FIFF.FIFFV_POINT_CARDINAL` landmarks.
 - Make `make_eeg_layout` still work even if no digitization points are present. Generate a warning though.
It used to be a dict. We made it a subclass with extras. Doc should be updates

> On 08 Nov 2014, at 21:20, Marijn van Vliet <notifications@github.com> wrote:
> 
> In mne/layouts/layout.py:
> 
> >      """Make a 2 dimensional sensor map from sensor positions in an info dict
> >  
> >      Parameters
> >      ----------
> > -    chs : list
> > -        A list of channels as contained in the info['chs'] entry.
> > +    info : dict
> > +        Measurement info
> > +    picks : list of int
> > +        Channel indices to generate topomap coords for
> I'm not going to fix all of your crummy docs ;-). periods at the end of sentences are wildly inconsistent throughout the documentation. I'll add periods on this file, but there really should be some consensus about them.
> 
> They thought me in coding school:
> sentence fragments should not end with a period nor start with a capital letter. Only valid English sentences should.
> 
> I agree with documenting default behaviour.
> 
> —
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