Add `sample_nifti.json` to `.gitignore`

[jwodder]

One of the tests creates a sample_nifti.json file in the heudiconv/tests/data/ directory, and this file needs to be in the same directory as some of the files that are already in the data/ directory (and so creating it in a temp directory won't work). This PR thus adds the file to .gitignore so that it will be ignored by Git.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (0c3f14b) 81.57% compared to head (764e998) 81.57%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #663 +/- ## ======================================= Coverage 81.57% 81.57% ======================================= Files 43 43 Lines 3929 3929 ======================================= Hits 3205 3205 Misses 724 724 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy)

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.

[yarikoptic]

I already merged some PR and caused conflict.

I am with a workaround for now, but ideally that test_get_key_info_for_fmap_assignment should be fixed - probably sample NIFTI file first copied to temp dir and then used in there. I missed the fact that we are modifying the tests directory - that makes heudiconv being untestable without write access to tests, which isn't good.

[jwodder]

@yarikoptic Conflict resolved, and issue #665 created for updating the test.

[github-actions[bot]]

:rocket: PR was released in v0.13.0 :rocket: