darts needs a data unregister hook
For instance tests/datawizard/unpartition
crashes with darts because some data gets unregistered, and darts should remove such data from its structures in that case. _starpu_data_set_unregister_hook
can be used for that, it just needs to be improved a bit to be able to register a list of hooks instead of just one, so that both starpu-mpi and darts can use it at the same time.