[FIX] Allow (but discourage) task, acquisition and run entities for coordsystem.json and electrodes.tsv

[effigies]

We've run into a situation where the regular validator regular expressions were written more laxly than the specification, leading to datasets that use entities that are against the spec but perfectly interpretable. In particular, the task entity has been permitted for coordsystem.json across EEG, iEEG, MEG and NIRS.

Following the pattern established in #1722, instead of marking these datasets invalid with a schema-based validator, we relax the spec but add warnings.

cc @bids-standard/raw-electrophys and @rob-luke @lpollonini (we should make a NIRS team...).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.06%. Comparing base (0e506f0) to head (c0915eb).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1888 +/- ## ======================================= Coverage 88.06% 88.06% ======================================= Files 16 16 Lines 1391 1391 ======================================= Hits 1225 1225 Misses 166 166 ```

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[effigies]

@dorahermes @VisLab WDYT?

[dorahermes]

Just noting that if a _task- entity is allowed for an _coordsystem.json it should also be allowed for an _electrodes.tsv.

[effigies]

Good catch. task, acquisition and run were all permitted in the legacy validator for both electrodes.tsv and coordsystem.json, and all six combinations exist on OpenNeuro. For consistency, I've updated this PR to treat these all the same.