Refactor out Assignment.update (replace with Assignment.update_flows) to prevent double-calculating flows

[InnovativeInventor]

This is a PR broken out from #372 to ease the code review process -- this is the second batch of PRs, after which that perf-tuning branch will be considered fully merged in.

[codecov-commenter]

Codecov Report

Merging #383 (572ed0e) into main (8acd44c) will decrease coverage by 0.01%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##             main     #383      +/-   ##
==========================================
- Coverage   88.17%   88.16%   -0.02%     
==========================================
  Files          37       37              
  Lines        1666     1664       -2     
==========================================
- Hits         1469     1467       -2     
  Misses        197      197              

Impacted Files	Coverage Δ
gerrychain/grid.py	73.49% <0.00%> (ø)
gerrychain/proposals/tree_proposals.py	27.27% <50.00%> (ø)
gerrychain/constraints/contiguity.py	96.59% <100.00%> (ø)
gerrychain/metagraph.py	94.44% <100.00%> (ø)
gerrychain/partition/assignment.py	95.89% <100.00%> (-0.11%)	⬇️
gerrychain/partition/partition.py	98.76% <100.00%> (ø)
gerrychain/proposals/proposals.py	97.67% <100.00%> (ø)
gerrychain/proposals/spectral_proposals.py	87.50% <100.00%> (ø)
gerrychain/updaters/compactness.py	97.72% <100.00%> (ø)
gerrychain/updaters/county_splits.py	100.00% <100.00%> (ø)
... and 3 more

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[gabeschoenbach]

Looks good!