Add ability to deterministically PRUNE particles from the current set by index

[fsaad]

• [ ] Add prune to trace_set.py
• [ ] Add prune to engine.py (four flavors)
• [ ] Add prune to inference.py (four flavors)
• [ ] Add prune inference_sps.py (four flavors)
• [ ] Add a test_prune to the test suite.