Add a method to create Compounds from coxeter shape objects

[chrisjonesBSU]

This PR adds a new loader method to conversion.py that generates an mBuild compound from a Coxeter shape object. Coxeter comes from the Glotzer lab, and is a great tool that generates vertices and edges of tons of different kinds of geometries ranging in complexity, which are converted to particles and bonds to create an mBuild Compound. Coxeter shapes provide relative coordinates, so the loader provides a scaling option to shift everything close enough to a target coordinate unit (which for mBuild is always nanometers). It also lets you set an element for the particles if desired, but this can be left blank so the particles are treated as generic beads.

Note: We might have to wait for a new release before we can use the coxeter conda-package as a dependency. I opened an issue asking about a new release timeline.

I still need to add unit tests

PR Checklist

---

• Includes appropriate unit test(s)
• Appropriate docstring(s) are added/updated
• Code is (approximately) PEP8 compliant
• Issue(s) raised/addressed?

[codecov]

Codecov Report

❌ Patch coverage is 16.00000% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.14%. Comparing base (52a0897) to head (e786acb).

Files with missing lines	Patch %	Lines
mbuild/conversion.py	4.76%	20 Missing ⚠️
mbuild/periodic_kdtree.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1353      +/-   ##
===========================================
- Coverage    82.37%   82.14%   -0.24%     
===========================================
  Files           64       64              
  Lines         6977     6999      +22     
===========================================
+ Hits          5747     5749       +2     
- Misses        1230     1250      +20     

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[chrisjonesBSU]

We might think about adding a to_coxeter method too, so we can take advantage of some of its analysis methods. Also, the coxeter shapes have a .is_inside() method that accepts a set of points, just like we have in constraints. We can add a .from_coexter() method to Constaint to create some interesting packing possibilities..