Avoid repeated status polling in smc

[aseyboldt]

Description

The smc progress indicator continually polls the status of the worker thread.
This instead uses mutliprocessing.wait to make this more efficient.

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks)
• If you are a pro: each commit corresponds to a relevant logical change

Type of change

• New feature / enhancement
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

---

📚 Documentation preview 📚: https://pymc--7351.org.readthedocs.build/en/7351/

[fonnesbeck]

LGTM

[fonnesbeck]

Re-starting failed test. Not clear its related to the change.

[aseyboldt]

The failure was due to the change, good thing we have tests :-)
Only the last finished chain was returned by the sampling function.