CollapseTest: Adding the ability to specify the initial H/He/H2/HM fraction in the ambient medium and the spheres. Metallicity in solar units is now user-specified.
New parameters: CollapseTestSphere[HII/HeII/HeIII/H2I]Fraction and CollapseTestInitialFraction[HII/HeII/HeIII/HM/H2I/H2II]
PhotonTest: Adding the ability to read in density and HII, HeII, and HeIII ionization fraction fields. Useful for post-processing radiative transfer problems.
PhotonTest: Calculate mean molecular weight from the species to initialize the gas energy (temperature) accurately.
I've addressed both Britton's and James' comments.
This PR passes the test suite. We're ready to rock.
This looks good to me!