Allow Interchange.combine when cutoffs differ by tiny amount

[mattwthompson]

Description

Resolves #1165. By default, Interchange.combine allows the vdW and Electrostatics cutoffs (and the switching distance) to differ by up to 1e-6 nm. The tolerance for 1-4 scaling is 1e-6 (unitless). These are not exposed to the user and I don't want them to change much, but I'm open to them being the wrong choices to start them out at.

Checklist

• Add tests
• Lint
• Update docstrings

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.85%. Comparing base (56af960) to head (7496c55).
⚠️ Report is 69 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1250      +/-   ##
==========================================
+ Coverage   93.57%   93.85%   +0.27%     
==========================================
  Files          72       72              
  Lines        6228     6232       +4     
==========================================
+ Hits         5828     5849      +21     
+ Misses        400      383      -17     

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[j-wags]

Awesome, LGTM!