Draft: Add OpenFoam tests for ubuntu 20.04

Janek Kozicki requested to merge foam20.04 into master

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.

Edited by Janek Kozicki

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