FIX: Permit electrodes.tsv in meg directories

[effigies]

Implements bids-standard/bids-specification#1555 for the legacy validator. RECOMMENDED status can wait on the schema.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -2.32 ⚠️

Comparison is base (268a44e) 85.71% compared to head (4c58017) 83.40%.

❗ Current head 4c58017 differs from pull request most recent head 1bf8ca1. Consider uploading reports for the commit 1bf8ca1 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1738      +/-   ##
==========================================
- Coverage   85.71%   83.40%   -2.32%     
==========================================
  Files         130       92      -38     
  Lines        6575     3893    -2682     
  Branches     1524     1270     -254     
==========================================
- Hits         5636     3247    -2389     
+ Misses        833      548     -285     
+ Partials      106       98       -8     

see 38 files with indirect coverage changes

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[sappelhoff]

placing this as a visual block, as we are still waiting for:

• [FIX] Clarify that electrodes.tsv is optional for MEG, for use with simultaneous (i)EEG bids-specification#1555

[sappelhoff]

see reasoning in:

• electrodes.tsv for simultaneous MEG/EEG data bids-specification#1550 (comment)