Fix CombineMotionParameters to support single-volume volume types

[tsalo]

Closes #310. This addresses a bug that will arise when a non-GE dataset is motion corrected, and one or more of the volume types in the ASL file only appear once (e.g., a single M0 scan in the ASL file).

Changes proposed in this pull request

• Allow mat_files inputs to CombineMotionParameters to be a single file- not just a list of files or None.
  • This is necessary because the MCFLIRT interface returns an OutputMultiPath, which returns a string if the list of outputs has only one element (e.g., if motion correction is run on a single volume). I hate OutputMultiPaths.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 🎉

Comparison is base (e4faea1) 62.89% compared to head (ebd91c6) 62.91%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #316      +/-   ##
==========================================
+ Coverage   62.89%   62.91%   +0.02%     
==========================================
  Files          56       56              
  Lines        5112     5113       +1     
  Branches      706      706              
==========================================
+ Hits         3215     3217       +2     
+ Misses       1650     1648       -2     
- Partials      247      248       +1     

Impacted Files	Coverage Δ
aslprep/interfaces/utility.py	88.13% <100.00%> (+0.06%)	⬆️

... and 1 file with indirect coverage changes

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