CI: turn coverage into own action

[junghans]

No description provided.

[codecov]

Codecov Report

Merging #562 into master will increase coverage by 32.3%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #562      +/-   ##
=========================================
+ Coverage    28.1%   60.4%   +32.3%     
=========================================
  Files         124     124              
  Lines        7622    7635      +13     
=========================================
+ Hits         2143    4613    +2470     
+ Misses       5479    3022    -2457     

Impacted Files	Coverage Δ
src/libcsg/boundarycondition.cc	100.0% <0.0%> (ø)
src/tools/csg_reupdate.cc	89.1% <0.0%> (+0.5%)	⬆️
...ibcsg/potentialfunctions/potentialfunctioncbspl.cc	90.9% <0.0%> (+1.6%)	⬆️
src/tools/csg_property.cc	83.8% <0.0%> (+4.7%)	⬆️
src/libcsg/modules/io/gmxtrajectoryreader.cc	81.9% <0.0%> (+4.9%)	⬆️
src/libcsg/nblistgrid.cc	96.1% <0.0%> (+7.6%)	⬆️
src/libcsg/modules/io/lammpsdumpreader.cc	74.8% <0.0%> (+9.2%)	⬆️
src/libcsg/csgapplication.cc	82.1% <0.0%> (+9.2%)	⬆️
include/votca/csg/csgapplication.h	57.8% <0.0%> (+15.7%)	⬆️
src/tools/csg_stat_imc.cc	79.4% <0.0%> (+16.9%)	⬆️
... and 56 more

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