Standard concentrations in the IdealMolalSolution phase depend on
a user-specified m_formGC parameter, where m_formGC=0 results in a
standard concentration of 1.0, m_formGC = 1 is supposed to result in
a standard concentration for species k equal to 1 divided by the
molar volume of species k, and m_formGC = 2 is supposed to result in
a standard concentration equal to 1 divided by the molar volume of the
solvent species (which is species 0).
Current behavior is that m_formGC = 1 and m_formGC = 2 *both* result
in a standard concentration of 1 divided by molar vlume of the solvent.
This commit fixes how this is handled, cleans up the switch statement
(the three cases were written somewhat inconsistently), and throws
an error if m_formGC is set < 0 or > 2.
3c9bbc4ec9