Rename `--freesurfer-input` to `--fs-subjects-dir`

[tsalo]

Closes #45 and closes #139.

Changes proposed in this pull request

• Rename the --freesurfer-input parameter to --fs-subjects-dir.
  • I noticed that QSIRecon was using fs_subjects_dir in places where it was probably supposed to be using freesurfer_input, so this almost certainly fixes some bugs.
• Explicitly check that the fs_subjects_dir exists in the parser. This should fix #139.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 33.52%. Comparing base (4495e9d) to head (95825cb).

Files with missing lines	Patch %	Lines
qsirecon/workflows/recon/anatomical.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #152 +/- ## ========================================== - Coverage 33.53% 33.52% -0.02% ========================================== Files 57 57 Lines 7117 7115 -2 Branches 965 965 ========================================== - Hits 2387 2385 -2 Misses 4628 4628 Partials 102 102 ```

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[tsalo]

Good question. I just looked and SUBJECTS_DIR isn't used anywhere except for the FreeSurferIngressTest.sh file.