Add bounds to sample_candidates in Constraints classes.

[chrisjonesBSU]

PR Summary:

This adds bounds parameters to volume constraints sample_candidates method. This lets you choose random starting points within a certain region, which adds more flexibility for different initialization.

This also changes volume.py to constraints.py to be more consistent, and it moves the file from mbuild/utils to path

PR Checklist

---

• Includes appropriate unit test(s)
• Appropriate docstring(s) are added/updated
• Code is (approximately) PEP8 compliant
• Issue(s) raised/addressed?
  …e file to match class names

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.85%. Comparing base (a84903f) to head (3f70e77).
⚠️ Report is 86 commits behind head on develop.

Files with missing lines	Patch %	Lines
mbuild/path/path.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1315   +/-   ##
========================================
  Coverage    78.85%   78.85%           
========================================
  Files           59       59           
  Lines         6284     6286    +2     
========================================
+ Hits          4955     4957    +2     
  Misses        1329     1329           

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[chrisjonesBSU]

Leaving this open for now as the region_minx and region_maxs behavior wasn't added in #1314 but we can do that in a separate PR, or merge it here.

[chrisjonesBSU]

I'm going to work on adding this in a separate PR rather than resolving the conflicts.