[CI] Codecov

[christophfroehlich]

• same settings file as of ros2_controllers
• add badge
• enable coverage build on pushes to master

fails until #128 is merged

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@6c0ea66). Click here to learn what that means.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #130   +/-   ##
=========================================
  Coverage          ?   45.80%           
=========================================
  Files             ?       12           
  Lines             ?      548           
  Branches          ?      287           
=========================================
  Hits              ?      251           
  Misses            ?       31           
  Partials          ?      266           

Flag	Coverage Δ
unittests	45.80% <0.00%> (?)

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