Fixes for EEGLAB: do not 'simplify cells' when loading.

[sappelhoff]

PR Description

I think I found the problem with #991

closes #991

@cmadjar it'd be great if you could try this fix. Install it by:

pip uninstall mne_bids
pip install pip install git+https://github.com/sappelhoff/mne-bids.git@use/mne/export

if you want to go back, simply uninstall mne_bids again and re-install the version you want.

I think the problem was that we used loadmat with simplify_cells, which changes the MATLAB "structure" and leads to problems when writing back.

In the long run it'd still be nicer to switch to "mne.export" for this.

Merge checklist

Maintainer, please confirm the following before merging.
If applicable:

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

[codecov]

Codecov Report

Merging #1039 (aa6e35a) into main (122f268) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1039      +/-   ##
==========================================
+ Coverage   95.17%   95.19%   +0.01%     
==========================================
  Files          25       25              
  Lines        3796     3805       +9     
==========================================
+ Hits         3613     3622       +9     
  Misses        183      183              

Impacted Files	Coverage Δ
mne_bids/copyfiles.py	97.71% <100.00%> (+0.09%)	⬆️

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[agramfort]

although I'll let @cmadjar confirms it fixes the problem

[hoechenberger]

I tried to simplify the docstring a little