ADIOS 2.1.0 (#7240)
ADIOS 2.1.0 (#7240)

Adds the first stable API release of ADIOS2, v.2.1.0.

Also adds a missing `pybind11` CMake flag for proper Python
selection (needed in both the new and previous version).
mfherbst/spack,krafczyk/spack,iulian787/spack,tmerrick1/spack,iulian787/spack,krafczyk/spack,EmreAtes/spack,mfherbst/spack,matthiasdiener/spack,matthiasdiener/spack,krafczyk/spack,matthiasdiener/spack,LLNL/spack,mfherbst/spack,LLNL/spack,tmerrick1/spack,tmerrick1/spack,EmreAtes/spack,tmerrick1/spack,iulian787/spack,EmreAtes/spack,matthiasdiener/spack,matthiasdiener/spack,mfherbst/spack,iulian787/spack,krafczyk/spack,LLNL/spack,tmerrick1/spack,EmreAtes/spack,krafczyk/spack,LLNL/spack,mfherbst/spack,EmreAtes/spack,iulian787/spack,LLNL/spack