Fix cut selection with belief and objective states

[odow]

Closes #892

Quite a bad correctness bug, but a bit tricky to find in simple tests!

Cuts get deleted, but because cut.belief_y was a pointer to node.belief_state.belief, if they later get added back and the belief has changed then they get added back in the wrong place.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.40%. Comparing base (3f6c839) to head (b3201e3).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #893      +/-   ##
==========================================
+ Coverage   96.37%   96.40%   +0.02%     
==========================================
  Files          28       28              
  Lines        4032     4035       +3     
==========================================
+ Hits         3886     3890       +4     
+ Misses        146      145       -1     

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