ADIOS2: Non-MPI Parallel Reads (DASK)

[ax3l]

For DASK-style parallel reading, we would like to avoid the overhead of opening and parsing file meta data for chunked, parallel reads.

This strategy is implemented already in MPI-parallel backends, where one rank does the open and parsing and broadcasts to all other ranks involved.

Exposing this to non-MPI parallel reads is started by ADIOS2 via ornladios/ADIOS2#3651 and we want to integrate it.