single step conditionals

[mr-c]

• fix the expression used by the output valueFrom hack
• add support for single-task conditionals

Inspired by #201

[codecov]

Codecov Report

Attention: Patch coverage is 96.77419% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 95.14%. Comparing base (5b40784) to head (3bdcfa8).

Files	Patch %	Lines
wdl2cwl/main.py	96.77%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #243      +/-   ##
==========================================
+ Coverage   94.35%   95.14%   +0.78%     
==========================================
  Files           7        7              
  Lines         850      926      +76     
  Branches      234      240       +6     
==========================================
+ Hits          802      881      +79     
+ Misses         21       18       -3     
  Partials       27       27              

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