MPI operators

Closes #42, #43, and #45

See merge request !16

fix examples times_of_interest, fix graph builder io_params, fix useless folder generation, fix argparser io_parms, add hdf5_disable_slicing, set io_params=False by default

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

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)