code coverage reporting with Codecov

[alexskr]

This PR:

1. adds cobertura code coverage formatting for simplecove which makes it injectable by codecov.
2. Adds github action step for uploading coverage reports to Codecov.io

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (develop@2ebad39). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop     #150   +/-   ##
==========================================
  Coverage           ?   80.83%           
==========================================
  Files              ?       63           
  Lines              ?     4867           
  Branches           ?        0           
==========================================
  Hits               ?     3934           
  Misses             ?      933           
  Partials           ?        0           

Flag	Coverage Δ
unittests	80.83% <0.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2ebad39...69425dc. Read the comment docs.