Add support for periodic and white boundary conditions

[paulromano]

This PR adds support for converting models with periodic and white boundary conditions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.86%. Comparing base (f2f9ae2) to head (54b2772).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
+ Coverage   85.69%   86.86%   +1.17%     
==========================================
  Files           3        3              
  Lines         720      731      +11     
  Branches      186      191       +5     
==========================================
+ Hits          617      635      +18     
+ Misses         55       52       -3     
+ Partials       48       44       -4     

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