move undeform_structure function to core utils

[tsmathis]

Small refactor to move the undeform_structures function out of the materials builder where it was floating outside the builder class as a helper function.

Moved to emmet-core utils to live with the other util functions.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 90.03%. Comparing base (9381cc2) to head (598e1c3).
Report is 1 commits behind head on main.

Files	Patch %	Lines
emmet-core/emmet/core/utils.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #999      +/-   ##
==========================================
- Coverage   90.03%   90.03%   -0.01%     
==========================================
  Files         142      142              
  Lines       13338    13337       -1     
==========================================
- Hits        12009    12008       -1     
  Misses       1329     1329              

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