Re-enable coveralls.

[MFraters]

We used to have coverage reports with the cig jenskins server. The goal of this pull request is to re-enable them. This may require some testing to get it working.

I discussed with @gassmoeller that we can just add it to the a current test and that we don't need a special tester for it.

[gassmoeller]

The github actions pipelines are not executed, because they report errors in the yaml file (https://github.com/geodynamics/aspect/actions/runs/6444260048). Can you check if that is related to the empty lines that you introduced between the steps?

[MFraters]

@gassmoeller I added lcov to the docker file as discussed.

[MFraters]

ah, sorry. Fixed it.