[MRG] _part- -> _split- to conform with BIDS

[sappelhoff]

see: bids-standard/bids-specification#460

to be merged before CIs pass:

• [MRG] split_naming='bids' changes from _part-%d to _split-%d mne-python#7794
  • backported to 0.20?
• _part- -> _split- for entity change bids-standard/bids-validator#960

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>"
• Commit history does not contain any merge commits

[sappelhoff]

appveyor is green, so I trust that Travis will also be green soon.

This is ready to be merged and then we can close this chapter :-)

[jasmainak]

why this random commit??! the stable branch doesn't yet work?

[sappelhoff]

The bids-validator releases very often (sometimes even twice a week), so I find it more "stable" for us to regularly pin to an up to date commit.

that particular commit corresponds to my merge of the split rules into the validator.

[codecov-commenter]

Codecov Report

Merging #417 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #417   +/-   ##
=======================================
  Coverage   94.37%   94.37%           
=======================================
  Files          11       11           
  Lines        1528     1528           
=======================================
  Hits         1442     1442           
  Misses         86       86           

Impacted Files	Coverage Δ
mne_bids/utils.py	98.52% <ø> (ø)
mne_bids/write.py	97.11% <ø> (ø)

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[jasmainak]

Thanks @sappelhoff !