Fix Lattice system class for coarse-grained molecules

[chrisjonesBSU]

This fixes #128 by adding a try/except statement to Molecule._align_backbones_z_axis in the case that the particles don't have elements.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.41%. Comparing base (7f85dc3) to head (92263ca).
Report is 1 commits behind head on main.

❗ Current head 92263ca differs from pull request most recent head 1246777. Consider uploading reports for the commit 1246777 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #129      +/-   ##
==========================================
+ Coverage   94.40%   94.41%   +0.01%     
==========================================
  Files          25       25              
  Lines        1840     1845       +5     
==========================================
+ Hits         1737     1742       +5     
  Misses        103      103              

Files	Coverage Δ
flowermd/base/molecule.py	96.83% <100.00%> (+0.06%)	⬆️
flowermd/base/system.py	92.00% <ø> (ø)