[ENH, MRG] Simplify reading dig

[alexrockhill]

Okay, this is the penultimate PR from #973. This simplifies the reading logic for digitizations to give a consistent warning for all templates. It also allows 'unknown' to be set as the coordinate frame for MEG data, which technically should be allowable because template coordinate frames are acceptable for MEG too. It gets rid of most of the specific references to 'Other' in reading which is my favorite part because that really shouldn't be a part of BIDS.

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>"

[alexrockhill]

After this, the plan is to do similar restructuring for write and add both read and write to the example. Modifying write is a bit trickier because sometimes we use a different space (usually Other) that then MEG/EEG/IEEG_coord_system which really needs to be fixed because these are supposed to be the same thing.

[codecov]

Codecov Report

Merging #979 (e84b520) into main (7343e20) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #979      +/-   ##
==========================================
+ Coverage   94.72%   94.88%   +0.15%     
==========================================
  Files          25       25              
  Lines        3698     3675      -23     
==========================================
- Hits         3503     3487      -16     
+ Misses        195      188       -7     

Impacted Files	Coverage Δ
mne_bids/config.py	97.43% <100.00%> (-0.13%)	⬇️
mne_bids/dig.py	95.39% <100.00%> (+2.59%)	⬆️

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[adam2392]

Ah much better. I like it.

[hoechenberger]

@adam2392 @alexrockhill
How confident are you that this is "safe" to merge just before the 0.10 release (which I had been planning to do tomorrow)?

[alexrockhill]

IMO, I would prefer to get it done before the release, maybe we can have @timonmerk @moritz-gerster test

[hoechenberger]

Alright, let's merge then once all is green!

[hoechenberger]

Thanks @alexrockhill!