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

Merge branch 'mpi-operators' of gricad-gitlab.univ-grenoble-alpes.fr:particle_methods/hysop into mpi-operators

Move back example interface outside hysop package (environment configuration by the interface without importing hysop)

Removed wrong usage of hysop main_comm, main_rank and main_size. Recommended usage is to use a mpi_param in operators.