Add gcovr config file

[ClausKlein]

It helps to simple do:

bash-5.2$ gcovr . 
lines: 17.2% (36 out of 209)
functions: 9.7% (3 out of 31)
branches: 7.8% (49 out of 628)
calls: 0.0% (0 out of 0)
bash-5.2$ 

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -3.04% ⚠️

Comparison is base (d19d892) 24.70% compared to head (18b5c97) 21.66%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
- Coverage   24.70%   21.66%   -3.04%     
==========================================
  Files           6        5       -1     
  Lines         251      240      -11     
  Branches      142      130      -12     
==========================================
- Hits           62       52      -10     
  Misses        181      181              
+ Partials        8        7       -1     

Flag	Coverage Δ
Linux	11.17% <ø> (-5.27%)	⬇️
Windows	21.42% <ø> (ø)

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see 1 file with indirect coverage changes

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