Patch for crash when applying 3D transform to 4D image

[coryshain]

When applying a 3D transform to a 4D image (intended: applying the same transform to every time slice), I get the following crash (relevant traceback only):

  File "/home/corys/parcellate/parcellate/model.py", line 253, in sample
    atlas_nii = nt.resampling.apply(
  File "/home/corys/anaconda3/envs/meg/lib/python3.9/site-packages/nitransforms/resampling.py", line 277, in apply
    resampled = asyncio.run(
  File "/home/corys/anaconda3/envs/meg/lib/python3.9/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/home/corys/anaconda3/envs/meg/lib/python3.9/asyncio/base_events.py", line 647, in run_until_complete
    return future.result()
  File "/home/corys/anaconda3/envs/meg/lib/python3.9/site-packages/nitransforms/resampling.py", line 111, in _apply_serial
    xfm_t = transform if n_resamplings == 1 else transform[t]
  File "/home/corys/anaconda3/envs/meg/lib/python3.9/site-packages/nitransforms/manip.py", line 64, in __getitem__
    return self.transforms[i]
IndexError: list index out of range

This patch tries to catch this use case by avoiding indexing into self.transform when the transform is 3D.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.10%. Comparing base (4c69fb4) to head (09e9248).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #236      +/-   ##
==========================================
- Coverage   95.32%   95.10%   -0.23%     
==========================================
  Files          28       28              
  Lines        2954     2939      -15     
==========================================
- Hits         2816     2795      -21     
- Misses        138      144       +6     

Flag	Coverage Δ
unittests	95.10% <100.00%> (+0.02%)	⬆️

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[effigies]

Lgtm. Thanks!