More precision in molar mass and univ. gas const.

Changed _molarMass from 28.97 to 28.96755 (effective contributions of
N2, O2, Ar, CO2, Ne, He, CH4, Kr, and H2; yes, everything beyond Ar is
trace).  Made a static const double universal gas constant as opposed to
a double literal.  Set the value (and increased accuracy) to 8.314462175
as opposed to simply 8.314.