Add tests and coverage

[mschoenlaub]

This PR adds codecov upload, coverage execution as well as initial integration tests. We do intentionally not tinker with documentation and setup.py-related issues here to not mix up concerns in this PR.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@b8faf64). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #13   +/-   ##
=========================================
  Coverage          ?   96.96%           
=========================================
  Files             ?        2           
  Lines             ?       33           
  Branches          ?        0           
=========================================
  Hits              ?       32           
  Misses            ?        1           
  Partials          ?        0           

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