[MRG] Fixes electrodes.tsv and coordsystem.json writing for EEG/iEEG to adhere to specification

[adam2392]

PR Description

Closes: #592

• electrodes and coordsystem files for EEG and iEEG data should only have subject/session/acquisition/space/suffix defined.
• simplified the arguments for _write_dig_bids using BIDSPath.

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>"

[sappelhoff]

LGTM except for CI failures and a minor comment, thanks Adam!

[codecov-io]

Codecov Report

Merging #604 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #604   +/-   ##
=======================================
  Coverage   92.74%   92.75%           
=======================================
  Files          17       17           
  Lines        2302     2305    +3     
=======================================
+ Hits         2135     2138    +3     
  Misses        167      167           

Impacted Files	Coverage Δ
mne_bids/dig.py	90.65% <100.00%> (+0.20%)	⬆️
mne_bids/write.py	95.51% <100.00%> (-0.01%)	⬇️

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[hoechenberger]

Thanks, @adam2392!