Support MPI parallelism in R2SManager

[paulromano]

Description

This PR makes changes in the R2SManager and related classes in order to work properly with MPI parallelism. The changes include:

• The neutron/photon transport steps are now executed via openmc.lib. Because the activation step uses mpi4py to decompose across materials, this implies that the Python script itself has to be called with mpiexec. Consequently, the neutron/photon transport steps cannot make a subprocess call to mpiexec ... openmc because nested mpiexec calls are are not supported.
• Some results have to be broadcast across ranks in order for the full workflow to work, and when results are written to file, they should only be written from one rank.
• Care has to be taken with temporary directories to ensure that all ranks see the same directory
• The TemporarySession context manager was also updated to work when called in parallel

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[pshriwise]

Some small suggestions mainly for clarity here, @paulromano, but it looks nice! Thanks for addressing this!

[paulromano]

@pshriwise thanks for your review! comments have been addressed

[pshriwise]

Thanks @paulromano! I'll give this a short 24h cooldown in case anyone else wants to have a look.