re-enable faster lower-d HDF5 output

• CarpetIOHDF5: Optimize lower-dimensional output -- only transfer necessary data to the I/O process

• CarpetIOHDF5: replace some assert(0) with CCTK_VERROR

Ticket is here: https://bitbucket.org/einsteintoolkit/tickets/issues/1853/diagonal-output-in-carpetiohdf5-disabled