Feature 718 pcp combine output names

[georgemccabe]

Pull Request Testing

• Describe testing already performed for these changes:
  

Added unit tests to ensure new functions behaved as expected. Modified an existing use case to test that I could add additional output field names and get a successful run. I had to set FCST_PCP_COMBINE_OUTPUT_NAME (1 name for each derivation specified), then I added FCST_PCP_COMBINE_EXTRA_NAMES , FCST_PCP_COMBINE_EXTRA_LEVELS, and FCST_PCP_COMBINE_EXTRA_OUTPUT_NAMES. The extra output names item also needed a name for each derivation specified.

• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  

Using the feature branch, test that these changes accomplish what you need to run your use case.

• Do these changes include sufficient documentation and testing updates? [Yes]

• Will this PR result in changes to the test suite? [No]
  

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[LoganDawson-NOAA]

The addition of FCST_PCP_COMBINE_EXTRA_OUTPUT_NAMES in this feature branch works with my use case that has multiple entries set for FCST_PCP_COMBINE_EXTRA_NAMES and FCST_PCP_COMBINE_EXTRA_LEVELS.