DSSP selection fixes (issue #1420)

[IAlibay]

Fixes #1420

Changes:

• Fixes a case where bond checking would return False on residues with only one atom.
• Undo uneccessary universe copies.
• Adds missing stable selection tests.

Developers certificate of origin

• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.26%. Comparing base (6c1ec8b) to head (2b87e32).
⚠️ Report is 108 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1438      +/-   ##
==========================================
- Coverage   95.13%   93.26%   -1.87%     
==========================================
  Files         165      165              
  Lines       12395    12419      +24     
==========================================
- Hits        11792    11583     -209     
- Misses        603      836     +233     

Flag	Coverage Δ
fast-tests	93.26% <100.00%> (?)
slow-tests	?

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[mikemhenry]

Just one small change, I think we should include the file hash

[github-actions]

No API break detected ✅