[clib] Enable access to critical properties of non-pure phases

Previously, only the critical properties of PureFluidPhase objects could be
accessed, but there are other ThermoPhase classes (specifically,
RedlichKwongMFTP) which overload the critTemperature, critPressure, and
critDensity functions.

git-svn-id: https://cantera.googlecode.com/svn/cantera/trunk@2392 02a645c2-efd0-11dd-984d-ab748d24aa7e