Align chains along z-axis when using system.Lattice

[chrisjonesBSU]

system.Lattice assumes that the molecule backbone is aligned down the z-axis of the box, then translates molecules along the x and y direction to build up the lattice. This doesn't work if the compounds aren't initially aligned down the z-axis. This PR adds a hidden method to Molecule which iterates over the mbuild compounds and translates them so that their backbone aligns with the z-axis. The backbone is defined as the line of best fit through the atomistic coordinates and uses cmelab/cmeutils#72 from cmeutils.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ab33ddd) 94.02% compared to head (54b7e37) 94.15%.
Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #107      +/-   ##
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+ Coverage   94.02%   94.15%   +0.12%     
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  Files          23       23              
  Lines        1724     1745      +21     
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+ Hits         1621     1643      +22     
+ Misses        103      102       -1     

Files	Coverage Δ
flowermd/base/molecule.py	96.77% <100.00%> (+0.19%)	⬆️
flowermd/base/system.py	91.95% <100.00%> (+0.05%)	⬆️
flowermd/library/polymers.py	100.00% <100.00%> (+1.33%)	⬆️