Draft: Add OpenFoam tests for ubuntu 20.04
We should add to the CI pipeline testing of OpenFoam coupling on ubuntu 20.04, since there appears to be some problem with this: https://answers.launchpad.net/yade/+question/694192
Currently only ubuntu 18.04 is tested: the CI job is named "test_18_foam".
EDIT: see my comment below for details.