Split giftis by hemisphere

[tsalo]

Closes #418.

Changes proposed in this pull request

• Modify the I/O spec JSON to include the hemi entity for functional GIFTI outputs.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.34%. Comparing base (94f8890) to head (bc5848d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #419   +/-   ##
=======================================
  Coverage   84.34%   84.34%           
=======================================
  Files          37       37           
  Lines        3986     3986           
=======================================
  Hits         3362     3362           
  Misses        624      624           

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