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 :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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