Create a dedicated job for coverage submission to codecov

[Algiane]

This PR allows to have two jobs that can be triggered independently in case of failure:

• the Mmg configuration, build and tests;
• the coverage submission.

As the coverage submission randomly fails with no reason, it allows to retry the submission from the github interface without running the entire Mmg tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.48%. Comparing base (3cf02ac) to head (ea83b7c).
Report is 13 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #262      +/-   ##
===========================================
+ Coverage    43.96%   50.48%   +6.52%     
===========================================
  Files          179      174       -5     
  Lines        54094    47106    -6988     
  Branches     10249    10254       +5     
===========================================
- Hits         23784    23783       -1     
+ Misses       22593    15605    -6988     
- Partials      7717     7718       +1     

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