Add html test-report

Add html report to lbmpy test runs (like pystencils). So human don't need to read terminal gibberish but can directly browse the html (e.g.: http://seitz.pages.walberla.net/-/lbmpy/-/jobs/416980/artifacts/test-report/index.html)

Might be a good idea to add this also to the god jobs in pystencils/lbmpy.

For some, reason the apt-get installed pytest does not like pytest-html