Add code coverage via Github Actions

[freemansw1]

Resolves #94 and adds code coverage via codecov.io and GitHub Actions.

We will want to merge this as a squash merge.

[freemansw1]

Apologies for spamming your email with the failed attempts here; this ended up being nontrivial.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (dev@ae378d2). Click here to learn what that means.
The diff coverage is n/a.

@@          Coverage Diff           @@
##             dev     #124   +/-   ##
======================================
  Coverage       ?   31.76%           
======================================
  Files          ?       11           
  Lines          ?     2043           
  Branches       ?        0           
======================================
  Hits           ?      649           
  Misses         ?     1394           
  Partials       ?        0           

Flag	Coverage Δ
unittests	31.76% <0.00%> (?)

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[freemansw1]

Note that we can turn those coverage comments off (I like them, and am happy for them to be on).

[JuliaKukulies]

Nice, well done @freemansw1! It looks all good to me. Yes, probably a good idea to squash the commits. I also like the comments and think it could be useful to leave them on. I assume that the issue with the missing base report will be automatically fixed when the PR is merged?

[freemansw1]

Thanks @JuliaKukulies! Yes, it should be the comparison version, but since this has never been run before, there's nothing to compare to.

[freemansw1]

I am inclined to go ahead and merge this after #141 is resolved and 1.3.1 is out, unless you think we should try to get it in for 1.3.1 @JuliaKukulies?

[JuliaKukulies]

I think that sounds like a good plan, especially when there will be an extra bug fix release 1.3.2, it can be released together with #140 as you suggested (and I think it would make sense and we do not need to wait for #140 to be reviewed to release 1.3.1).