Make sure output is treated consistently in R2SManager

[paulromano]

Description

Small PR here to fix unwanted output behavior when carrying out R2S runs with R2SManager:

• Before, an initialization of an empty OpenMC model (from openmc.lib.TemporarySession) was shown. This is fixed by passing output=False to TemporarySession
• The actual initialization of the neutron transport model was always shown regardless of the output option specified in run_kwargs. Now it is handled consistently: if the user calls manager.run(run_kwargs={'output': True}) they will see output and if they call manager.run(run_kwargs={'output': False}) they will not see output.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) 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)

[GuySten]

Looks good to me.

[pshriwise]

Looks good to me as well @paulromano. Thanks!