Adjusted variable names for clarity and codespell false positives

[TheChymera]

Codespell currently fails due to false positive detection of “inports”, which are input ports and not a misspelling of imports.
Also makes the code more clear I think.

[TheChymera]

Not sure the same sort of fix is good for “expad” (similarly ex-padding), because that's a verbatim parameter for AFNI... probably not a good idea to introduce an underscore there.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (5ef9fe4) 63.15% compared to head (71e8795) 63.15%.

Files	Patch %	Lines
nipype/pipeline/engine/utils.py	78.94%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #3627    +/-   ##
========================================
  Coverage   63.15%   63.15%            
========================================
  Files         308      308            
  Lines       40825    40825            
  Branches     5656     5500   -156     
========================================
  Hits        25781    25781            
  Misses      14031    14031            
  Partials     1013     1013            

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