Fix multiprocessing Pool shutdown issue on Python 3.13+

[zblz]

Summary

• Fixes the OSError: [Errno 9] Bad file descriptor that occurs on Python 3.13+ when the multiprocessing Pool created in get_sampler is garbage-collected without being properly shut down
• Creates the pool separately and skips it entirely for single-threaded runs (threads=1), passing pool=None to emcee (which runs serially)
• Attaches the pool reference to the sampler via _naima_pool and explicitly calls close()/join() after run_sampler completes

Closes #245