Allow None when setting the state of a PureFluid

Allow None as an option when setting the temperature and quality or
pressure and quality of a PureFluid. This mimics similar functionality
for Solutions.

git-svn-id: https://cantera.googlecode.com/svn/cantera/trunk@3213 02a645c2-efd0-11dd-984d-ab748d24aa7e
[Cython] Allow None when setting the quality of a PureFluid

Allow None as an option for the quality as well as the
temperature and pressure when setting the state of a PureFluid.
Completes r3213.

git-svn-id: https://cantera.googlecode.com/svn/cantera/trunk@3214 02a645c2-efd0-11dd-984d-ab748d24aa7e