Fix: Extract unit information from image header in CompCor

[eort]

Fixes #2456 .

Changes proposed in this pull request

• extract unit information from image header, rather than from image directly

[effigies]

LGTM

[codecov-io]

Codecov Report

Merging #2458 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #2458   +/-   ##
=======================================
  Coverage   66.62%   66.62%           
=======================================
  Files         328      328           
  Lines       42508    42508           
  Branches     5280     5280           
=======================================
  Hits        28321    28321           
  Misses      13505    13505           
  Partials      682      682

Flag	Coverage Δ
#smoketests	50.73% <0%> (ø)	⬆️
#unittests	63.95% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
nipype/algorithms/confounds.py	67.31% <0%> (ø)	⬆️

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