Add third config to BIDSLayout that defines cohort entity

[tsalo]

Closes none. The BIDSLayout with bids and derivatives configs was not identifying cohort, even though I had invalid_filters="allow". I thought that invalid_filters="allow" would let BIDSLayout find any entity-formatted strings in filenames, but apparently that's not true.

Changes proposed in this pull request

• Add a new pybids config file that include cohort, but not datatype (which conflicts with the existing datatype definition).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.26%. Comparing base (13eff94) to head (7a74864).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1143   +/-   ##
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  Coverage   83.26%   83.26%           
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  Files          53       53           
  Lines        6667     6669    +2     
  Branches      924      924           
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+ Hits         5551     5553    +2     
  Misses        907      907           
  Partials      209      209           

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