[BUG, ENH, MRG] Pixels

[alexrockhill]

Implements part of the fix from here: #973

This both adds support for ieeg data in pixels and prevents montages from being identity mapped to head when it's not appropriate. Before pixels data was not read in whereas "Other" coordinate frame data was. I see no reason MNE users can't analyze data in pixels, they just have to handle the coordinate frame themselves and have it set as "unknown" like the rest of the unsupported coordinate frames. This accompanies the issue that when the coordinate frame was "unknown" in a BIDS dataset it was mapped to head with an identity transform. I think best just to leave these in "unknown" so that people aren't confused that it's in head for some reason when it really isn't.

Merge checklist

Maintainer, please confirm the following before merging:

• All comments resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated
• PR description includes phrase "closes <#issue-number>"

[codecov]

Codecov Report

Merging #976 (2bda182) into main (048bd0e) will increase coverage by 0.06%.
The diff coverage is 100.00%.

❗ Current head 2bda182 differs from pull request most recent head 6d42709. Consider uploading reports for the commit 6d42709 to get more accurate results

@@            Coverage Diff             @@
##             main     #976      +/-   ##
==========================================
+ Coverage   94.57%   94.63%   +0.06%     
==========================================
  Files          25       25              
  Lines        3597     3600       +3     
==========================================
+ Hits         3402     3407       +5     
+ Misses        195      193       -2     

Impacted Files	Coverage Δ
mne_bids/config.py	97.46% <ø> (ø)
mne_bids/dig.py	91.37% <100.00%> (+1.16%)	⬆️

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[alexrockhill]

Ok, this should be good to go once everything comes back green, and allowing for pixels is 80% of the way there to allowing all the template coordinate frames since it sets the coordinate frame to unknown instead of head

[agramfort]

otherwise LGTM

[hoechenberger]

Thanks @alexrockhill!