Net decomposition: tuning and polishing

[duck2]

Paper version of the net decomposing router. Major fixes are:

1. Don't count pruned sinks towards high fanout case's chan_nodes_added
2. Add a minimum bin size for the HF case to prevent minuscule bins when flat routing is enabled
3. Incrementally update the PartitionTree instead of rebuilding on every iteration

[vaughnbetz]

Also add tests for the net decomposing router. One small circuit in strong, so it is checked all the time. Should have a run on bigger designs too in one of the nightly tests.

[vaughnbetz]

Should try to land this before the code format PR if at all possible. Fahri thinks this can be landed in 2 weeks (mid-November) but that it shouldn't block a code format PR.

[vaughnb-cerebras]

@duck2 : can you add the QoR data as it becomes available? We'll need: net-decomposing router, serial router, master router anyway (your call if you also want to check the baseline router to be safe)