Update Statsframe Graph after DataFrame Squash

[michaelmckinsey1]

• In Fix filter_stats #148, we made a change to enable clearing the StatsFrame during squash to update it from the new version of the Thicket DataFrame. However, this does not make sense in certain cases (as in filter_stats), as it will clear the computed statistics in the statsframe.dataframe (ref https://github.com/LLNL/thicket/pull/148/files#diff-deda0d817c7bf26adbe7b20238b4eb4b92535a8b08b047a20f291d83041f74f8L1082). Therefore, we need to manually reset the statsframe graph to the Thicket graph.
• Add unit test

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.07%. Comparing base (8c7e4be) to head (c754c2e).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #263   +/-   ##
========================================
  Coverage    83.07%   83.07%           
========================================
  Files           65       65           
  Lines         4501     4503    +2     
========================================
+ Hits          3739     3741    +2     
  Misses         762      762           

Files with missing lines	Coverage Δ
thicket/tests/test_filter_stats.py	90.00% <100.00%> (+0.34%)	⬆️
thicket/thicket.py	89.06% <100.00%> (+0.01%)	⬆️

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