Add optional dir entity to T2starmap pattern

[tsalo]

I am currently processing multi-echo BOLD data with both AP and PA acquisitions through fMRIPrep, and I noticed that the T2starmap outputs didn't include the direction entity, so I was only getting one of the two directions for each acquisition.

I also made task optional for T2starmap, but I can revert that.

[effigies]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.44%. Comparing base (87319fe) to head (359c05c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #898   +/-   ##
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  Coverage   71.44%   71.44%           
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  Files          87       87           
  Lines        8402     8402           
  Branches      829      829           
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  Hits         6003     6003           
  Misses       2394     2394           
  Partials        5        5           

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