Handle outputs with multiple output sources

[simleo]

E.g.:

outputs:
  out_file:
    type: File
    outputSource:
      - date/date_out
      - isodate/date_out
    pickValue: first_non_null

Similar to #31.

This fixes the error that was triggered by the RO bundle provided in #33 after #38 fixed the original one (see #38). Unfortunately the conversion of that bundle still fails, this time with:

RuntimeError: packed.cwl#workflow_prepare_fasta_db.cwl has no corresponding action

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.06 🎉

Comparison is base (8ab22b7) 97.19% compared to head (56dd1a7) 97.25%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
+ Coverage   97.19%   97.25%   +0.06%     
==========================================
  Files          11       11              
  Lines        1746     1786      +40     
==========================================
+ Hits         1697     1737      +40     
  Misses         49       49              

Impacted Files	Coverage Δ
src/runcrate/convert.py	97.56% <100.00%> (+<0.01%)	⬆️
tests/test_cwlprov_crate_builder.py	99.87% <100.00%> (+<0.01%)	⬆️

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