Add coverage to CI

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (setup-ci@2538e29). Click here to learn what that means. The diff coverage is n/a.

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@@             Coverage Diff             @@
##             setup-ci     #164   +/-   ##
===========================================
  Coverage            ?   60.75%           
===========================================
  Files               ?      105           
  Lines               ?    18646           
  Branches            ?        0           
===========================================
  Hits                ?    11329           
  Misses              ?     7317           
  Partials            ?        0           

Flag	Coverage Δ
unittests	60.75% <0.00%> (?)

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[mmore500]

Looks like our current test coverage is 60%

https://app.codecov.io/gh/jeetsukumaran/DendroPy/pull/164/tree