Add coverage check to CI

[ambrussimon]

• exclude remaining uncovered lines to get to 100%
  • at least with a custom comment for grepping
  • preferably with easy way of still running coverage disregarding these "artificial exclusions"
• add final check for 100% coverage after tests

[nagem]

This option was reviewed and discussed in PR #891. It was decided we would not move forward with this type of solution.