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 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.21%. Comparing base (53013f1) to head (c5e4984).

Files with missing lines	Patch %	Lines
mbuild/simulation.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1315   +/-   ##
========================================
  Coverage    82.20%   82.21%           
========================================
  Files           59       59           
  Lines         6283     6285    +2     
========================================
+ Hits          5165     5167    +2     
  Misses        1118     1118           

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