Ensure length of flattened_data is same as end of cumulative_length

[iguinn]

• Fixed bug where _set_vector_unsafe was unsafely failing to resize the array
• Emit a warning in the constructor if the flattened data has a different size than it should, and then resize it to match cumulative_length
• Added a test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.82%. Comparing base (f6fb8ef) to head (31e1886).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #211   +/-   ##
=======================================
  Coverage   80.81%   80.82%           
=======================================
  Files          47       47           
  Lines        3978     3980    +2     
=======================================
+ Hits         3215     3217    +2     
  Misses        763      763           

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