FIX: FSL orthogonalization bug

[effigies]

@jokedurnez This is the fix you suggested in #2073. I'm reading the second comment in that thread as verification that it successfully did what you intended, but I just want to double-check.

Fixes #2073.

Changes proposed in this pull request

• Cherry-picked @jokedurnez's commit
• Minor refactor to reduce number of times the same string is calculated, and satisfy PEP8

[codecov-io]

Codecov Report

Merging #2523 into master will decrease coverage by <.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master    #2523      +/-   ##
==========================================
- Coverage   66.88%   66.88%   -0.01%     
==========================================
  Files         327      327              
  Lines       42479    42482       +3     
  Branches     5268     5269       +1     
==========================================
  Hits        28413    28413              
- Misses      13365    13367       +2     
- Partials      701      702       +1

Flag	Coverage Δ
#smoketests	50.84% <75%> (ø)	⬆️
#unittests	64.22% <50%> (-0.02%)	⬇️

Impacted Files	Coverage Δ
nipype/interfaces/fsl/model.py	80.42% <75%> (-0.04%)	⬇️
nipype/interfaces/spm/base.py	86.79% <0%> (-0.67%)	⬇️

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