MSContactor and phase_frac

[dallan-keylogic]

Presently, the MSContactor model adds phase fraction variables whether or not they already exist on a StateBlock. As a result, it cannot be used with property packages that already have a phase_frac variable, such as the entire modular property framework. All that's needed to fix this issue is to add a check to see if phase_frac already exists (this approach is already implemented in the ControlVolume blocks).