Add an initial codecov.yml file to tell it to ignore test and examples

[tleonhardt]

Add an initial codecov.yml file to tell Codecov to ignore test and examples code.

Also tell it to split the plugin directory into a separate component.

Closes #1436

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.54%. Comparing base (dcda271) to head (7bda7f9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1437      +/-   ##
==========================================
+ Coverage   98.15%   98.54%   +0.39%     
==========================================
  Files          41       20      -21     
  Lines       13744     5723    -8021     
==========================================
- Hits        13490     5640    -7850     
+ Misses        254       83     -171     

Flag	Coverage Δ
unittests	98.54% <ø> (+0.39%)	⬆️

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