[BugFix] Fixes and improvements to H-He EOS

[adamdempsey90]

Background

• Does a better job of partition function calculations for molecular hydrogen.
• Corrects a bug where I was using the non-equilibrium expression for the H2 energy, as well.
• Correctly handle X=0 and Y=0 cases.

Contains all of the changes in the unit testing MR. The EOS unit test was very helpful in debugging and checking everything was working. I have improved it some more.

Description of Changes

Checklist

• New features are documented
• Tests added for bug fixes and new features
• (@lanl.gov employees) Update copyright on changed files