SpectroscopicQoI molecular weights

[tradowsk]

The Spectroscopic Absorption QoI (PR #46) does not account for the different molecular weights of isotopologues, and instead uses the weighted average provided by Antioch/Cantera.

Calculations have shown that the effect of accounting for the multiple molecular weights affects the final QoI value by 0.0066%.

This issue is here for reference for future work.