Fix automatic_density_by_lengths and add tests for it

[janosh]

Closes #3217.

[codecov-commenter]

Codecov Report

Patch coverage: 50.00% and project coverage change: -0.01% ⚠️

Comparison is base (bdab025) 72.28% compared to head (509a32d) 72.28%.

❗ Current head 509a32d differs from pull request most recent head 21f870c. Consider uploading reports for the commit 21f870c to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3218      +/-   ##
==========================================
- Coverage   72.28%   72.28%   -0.01%     
==========================================
  Files         238      238              
  Lines       70253    70252       -1     
  Branches    16332    16333       +1     
==========================================
- Hits        50785    50783       -2     
- Misses      16377    16378       +1     
  Partials     3091     3091              

Files Changed	Coverage Δ
pymatgen/io/vasp/inputs.py	85.71% <50.00%> (-0.02%)	⬇️

... and 2 files with indirect coverage changes

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