CarpetIOHDF5: create outdir for sliced output on every single ioproc
this is required in case there is no shared file system (or in the very
unlikley scenario where one IOProc is is so fast it wants to write out
before root managed to create the output directory).
CarpetIOHDF5: number files consecutively even when ioproc_every != 1
so far they are numbered by the rank of their IOProc which confuses the
readers incl. Carpet's file reader, and VisIt
Ticket is https://trac.einsteintoolkit.org/ticket/1977