Adding Single Chain system class

[StephMcCallum]

• doc strings
• add to library/systems.py
• make it work for AA atom models and atom indices (different ways to get length of polymer or mbuild compound, spicy.pdist)
• unit tests

[chrisjonesBSU]

This is a good start! I left a few comments for changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.78%. Comparing base (a600b73) to head (0dcd4e3).
Report is 65 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #192      +/-   ##
==========================================
+ Coverage   94.77%   94.78%   +0.01%     
==========================================
  Files          26       27       +1     
  Lines        1970     2072     +102     
==========================================
+ Hits         1867     1964      +97     
- Misses        103      108       +5     

Files with missing lines	Coverage Δ
flowermd/library/__init__.py	100.00% <100.00%> (ø)
flowermd/library/systems.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[chrisjonesBSU]

Couple more comments. We just need some unit tests now.