add github workflow actions for running tests; fixes #50

[thurber]

This update will cause Linux builds/tests to occur on any push, and OSX and Windows builds/tests to occur on any Pull Request.

FYI openjournals/joss-reviews#3313

[thurber]

i think the NOAA servers are struggling, so the download test is failing... will revisit this in a day or so

[codecov]

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@@            Coverage Diff            @@
##             master      #52   +/-   ##
=========================================
  Coverage          ?   97.31%           
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  Files             ?       19           
  Lines             ?      708           
  Branches          ?        0           
=========================================
  Hits              ?      689           
  Misses            ?       19           
  Partials          ?        0           

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[thurber]

okay @amandadsmith, @crvernon, after the prerequisite battle with building on windows, we now have continuous integration testing on commits and pull requests!