Testing: Codecov uploader update

[marshallward]

This patches updates the codecov uploader to the new version, replacing
the bash uploader to be phased out in 2022.

The uploader URL is now a configuable variable, and the coverage scripts
have been modified to accommodate changes in the uploader.

Under this new uploader, the coverage reports must be generated locally,
rather than relying on the bash script to call gcov. So we now generate
these scripts based on the *.gcda output.

[codecov]

Codecov Report

Merging #21 (0eb5378) into dev/gfdl (ad35e4a) will increase coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 0eb5378 differs from pull request most recent head 7d80abc. Consider uploading reports for the commit 7d80abc to get more accurate results

@@            Coverage Diff            @@
##           dev/gfdl      #21   +/-   ##
=========================================
  Coverage     29.17%   29.17%           
=========================================
  Files           240      240           
  Lines         71493    71491    -2     
=========================================
  Hits          20856    20856           
+ Misses        50637    50635    -2     

Impacted Files	Coverage Δ
src/framework/MOM_domains.F90	51.72% <0.00%> (+0.87%)	⬆️

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[Hallberg-NOAA]

These changes look sensible to me, and this has passed the TC testing. In this particular case there is no point in running the pipeline tests on these changes, so the TC testing alone will suffice.