Formula for `Mv` using fractional abundances of the isotopes. $$M_v = 2\bar{M}_H + \bar{M}_O$$ Look at: [Hayes 2004](https://web.archive.org/web/20220629123450/https://web.gps.caltech.edu/~als/research-articles/other_stuff/hayes-2004-3.pdf) Change taken from #1782, needs new tests.
Formula for
$$M_v = 2\bar{M}_H + \bar{M}_O$$
Mvusing fractional abundances of the isotopes.Look at:
Hayes 2004
Change taken from #1782, needs new tests.