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

safer --clean for examples, now prompts the user to confirm action. Added no interactive option to force cleaning.

Re-enable HDF5 parallel interface. (reverting commit 9c195f33). It works only in a few specific cases (1d topology + cuting direction in the last varying direction)

Disable parallel hdf5 interface. In parallel simulations, one h5 file is written by each process, and joined through xmf 'JOIN' function on dataitems. It can be enabled with WITH_PARALLEL_HDF5 cmake flag