IAPWS - mass basis and update htpx function

[jghouse88]

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For the IAPWS property package, it will be useful to have the following:

• [ ] mass basis in addition to the existing mol basis (or some functionality that leverages the units framework)
• [X] htpx function currently requires T and P or vapor fraction. It will be useful to extend this to return enthalpy as a function of P,x.

[eslickj]

The htpx function should work with P, x now, so we are down to adding a mass-based option. This would just be an option where state variables are enthalpy is in J/kg and flow is in kg/s. Otherwise mass basis quantities should exist, but I need to double check all those to be sure nothing is missing.

[eslickj]

This is done in PR #1000!