Eliminate several member variables which shadow variables of the
VPStandardState class, and actually contained the same information
calculated a different way.

In some models, SpeciesThermoInterpType objects on individual species are not
used. Instead of requiring the specification of placeholder thermo data, this
allows the base SpeciesThermoInterpType class to be used, which will throw an
exception if it is inadvertently used.

Since VPStandardStateTP and derived classes do not use the reference state
thermodynamic properties in the m_spthermo object, we can just install
placeholder objects there, and eliminate the wrapper clas STITbyPDSS.

PDSS objects do not interact with the MultiSpeciesThermo object for a phase,
but at most a single SpeciesThermoInterpType object.