MRG: Remove unused argument and refactor

[larsoner]

I think we have some code path cruft that has built up in mne/viz/topomap.py.

1. image_mask is never used to modify the plots, so I removed it.
2. _make_image_mask both set up the (unused) image_mask and modified pos based on autoshrink, so I renamed it to _autoshrink.
3. outlines gets set by _check_outlines to always be a dict. So I removed the conditionals having to do with if outlines is None that happen afterward.
4. Moves _plot_corrmap to mne/viz/topomap.py to keep plotting code in mne/viz

This is a step toward #5085. Simplifying this first and getting feedback should make the subsequent steps easier.

[larsoner]

@jona-sassenhagen do you have time to take a look?

[codecov]

Codecov Report

Merging #5140 into master will increase coverage by 0.02%.
The diff coverage is 82.08%.

@@            Coverage Diff             @@
##           master    #5140      +/-   ##
==========================================
+ Coverage   88.14%   88.17%   +0.02%     
==========================================
  Files         357      358       +1     
  Lines       65777    65902     +125     
  Branches    11192    11208      +16     
==========================================
+ Hits        57982    58111     +129     
+ Misses       4958     4956       -2     
+ Partials     2837     2835       -2

[larsoner]

This is verbatim copy-paste except for the removal of image_mask=None here

[jona-sassenhagen]

So it seems it was originally used for this:

    if _is_default_outlines and image_mask is None:
        # prepare masking
        image_mask, pos = _make_image_mask(outlines, pos, res)

    if image_mask is not None and not _is_default_outlines:
        Zi[~image_mask] = np.nan

Hm.

I guess I approve.

[larsoner]

Ahh. If you are using the default (_is_default_outlines), then you won't ever enter the second conditional. The only way you do is if you aren't using the default outlines, and you have supplied an image_mask. This has to be the same size as Zi. This does not seem easy to achieve for the user... I think it's okay, then.

[massich]

LGTM