Feature template with hpp

[hsoh-u]

Expected Differences

Some linkers might have problems on generating instances of the template functions from the library files.
The template functions in cc files are separated to *hpp and *.hpp are included at *.h".

• Do these changes introduce new tools, command line arguments, or configuration file options? [No]
  
  If yes, please describe:
  

• Do these changes modify the structure of existing or add new output data types (e.g. statistic line types or NetCDF variables)? [No]
  
  If yes, please describe:
  

Pull Request Testing

• Describe testing already performed for these changes:
  

Build MET on various hosts (including test task).

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

• Do these changes include sufficient documentation updates, ensuring that no errors or warnings exist in the build of the documentation? [No]

• Do these changes include sufficient testing updates? [No]

• Will this PR result in changes to the test suite? [No]
  
  If yes, describe the new output and/or changes to the existing output:
  

• Please complete this pull request review by [Fill in date].
  

Pull Request Checklist

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• Complete the PR definition above.
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[jprestop]

I reviewed the code changes and tested the compilation of feature branch and the successful runs of "make test" on Hera, Jet, and Orion. The tests have all passed in this PR. I approve this request. Thanks for all of your work @hsoh-u.