Add units tab to docs

[jennyfothergill]

PR Summary:

With a little more work this fixes #137

• distance is in nm
• energy is in kJ/mol
• angles are radians
• harmonic bonds are defined with a factor of 0.5 in front.
• Include the simple conversions from opls-style fourier series to RB.

related to mosdef-hub/mbuild#858 (The links will not work until both are merged)

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

Merging #402 (1cacf63) into master (2db95ac) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #402   +/-   ##
=======================================
  Coverage   71.28%   71.28%           
=======================================
  Files          16       16           
  Lines        1508     1508           
=======================================
  Hits         1075     1075           
  Misses        433      433           

[umesh-timalsina]

General comments. Looks Good.

[jennyfothergill]

it seems like without the readthedocs.yml the build is no longer triggered in the PR. @umesh-timalsina can you fix this in the rtd dashboard?

[umesh-timalsina]

@jennyfothergill. It should be now.