Making workflow step run field expansion something that can be turned off

[kellrott]

Also fixes a few assorted bugs.

[codecov[bot]]

Codecov Report

Merging #6 into master will decrease coverage by 0.64%. The diff coverage is 18.18%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
- Coverage   61.52%   60.87%   -0.65%     
==========================================
  Files           5        5              
  Lines         473      478       +5     
==========================================
  Hits          291      291              
- Misses        182      187       +5

Impacted Files	Coverage Δ
cwlgen/workflow.py	23.8% <0%> (-0.59%)	:arrow_down:
cwlgen/import_cwl.py	66.22% <25%> (-0.89%)	:arrow_down:

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[khillion]

Thanks!