Fixed prefix of input union types

[LanderOtto]

This commit fixes two problems:

• The extend built-in method of the List was used, but a return value was attended. However, the extend method operates in-place.
• When a prefix is present in an input that has union types, the prefix was duplicated several times, e.g. --input --input DATA. The CWL inputBinding definition used to be evaluated both in the internal CWLCommandToken objects of the CWLUnionCommandToken, and in an external CWLCommandToken wrapping the CWLUnionCommandToken. Now, the external CWLCommandToken is not created and it is used the CWLUnionCommandToken directly.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 70.14%. Comparing base (244de3b) to head (958b5b2).
Report is 4 commits behind head on master.

Files	Patch %	Lines
streamflow/cwl/command.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #455      +/-   ##
==========================================
- Coverage   70.18%   70.14%   -0.04%     
==========================================
  Files          83       83              
  Lines       10587    10591       +4     
  Branches     2505     2508       +3     
==========================================
- Hits         7430     7429       -1     
- Misses       2702     2703       +1     
- Partials      455      459       +4     

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