ENH: Only write FLIRT log if not previously written

[effigies]

Changes proposed in this pull request

• Add FLIRT._log_written flag to indicate whether to write the log. This prevents multiple calls to aggregate_outputs from cluttering the log.
• Some additional logic/style fixes in _parse_inputs while I was there.
  • Don't overwrite a pre-existing skip list
  • Take advantage of undefined traits evaluating False

[codecov-io]

Codecov Report

Merging #2069 into master will increase coverage by <.01%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master    #2069      +/-   ##
==========================================
+ Coverage   72.16%   72.17%   +<.01%     
==========================================
  Files        1137     1137              
  Lines       57194    57195       +1     
  Branches     8194     8193       -1     
==========================================
+ Hits        41276    41279       +3     
+ Misses      14629    14627       -2     
  Partials     1289     1289

Flag	Coverage Δ
#smoketests	72.17% <77.77%> (ø)	⬆️
#unittests	69.78% <44.44%> (ø)	⬆️

Impacted Files	Coverage Δ
nipype/interfaces/fsl/preprocess.py	82.6% <77.77%> (+0.02%)	⬆️
nipype/interfaces/base.py	83.99% <0%> (+0.18%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a63c52d...6b08c27. Read the comment docs.