workflow level non-bound WDL declarations

[mr-c]

• support fixed value workflow variable declarations
• add logical, less-than functions

Fixes #147

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 94.70%. Comparing base (74c7737) to head (4d367f0).

Files	Patch %	Lines
wdl2cwl/main.py	72.72%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #247      +/-   ##
==========================================
- Coverage   94.90%   94.70%   -0.21%     
==========================================
  Files           7        7              
  Lines         943      945       +2     
  Branches      245      245              
==========================================
  Hits          895      895              
- Misses         20       21       +1     
- Partials       28       29       +1     

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