Add node to check for subject's Freesurfer directory

[tsalo]

Closes #503.

Changes proposed:

• Add a node to init_surface_recon_wf that is only called if --fs-no-resume is used. This node checks if the subject's Freesurfer directory exists and raises an error if it does not.

[effigies]

I would suggest that we gate the other nodes on this passing, so have it do something like:

fs_params = [(field, field) for field in ('subjects_dir', 'subject_id')]
workflow.connect([
  (inputnode, check_subjects_dir, fs_params),
  (check_subjects_dir, fs_base_inputs, fs_params),
  (check_subjects_dir, get_surfaces, fs_params),
  (check_subjects_dir, save_midthickness, [
      ('subjects_dir', 'base_directory'),
      ('subject_id', 'container'),
  ]),
])

IDC if you actually split out fs_params, I just didn't want to write it a bunch. Also, I'm realizing you could go ahead and collapse get_surfaces into fs_base_inputs/autorecon1, but that's a bit out of scope, so no worries if you don't want to.

[tsalo]

Sure, I can do that.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.77%. Comparing base (da4ee07) to head (fe337a8).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #504      +/-   ##
==========================================
+ Coverage   70.88%   74.77%   +3.88%     
==========================================
  Files          25       25              
  Lines        2068     2077       +9     
  Branches      268      270       +2     
==========================================
+ Hits         1466     1553      +87     
+ Misses        544      456      -88     
- Partials       58       68      +10     

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