Fix ci cleanup in docker image (pip uninstall not works without pythonpath export and do not remove INSTALLDIR but INSTALLDIR/lib/site-package/*). Various fixes

take into account args.npts in sediment examples (for ci) and added a limit of one candidate opencl kernel

add docker focal image, fix mpi regression for allowing allow-run-as-root from environment variables (https://github.com/open-mpi/ompi/issues/6920)