Bug in modular properties bubble and dew point calculations

[andrewlee94]

A user brought up an issue in the support mailing list where they were trying to model a three phase system (VLS) with VLE (no solids equilibrium) and were seeing the incorrect VLE results if solids were present.

After digging into the model a little, I found that the dew point was changing depending on the solid concentration and looking at the dew point constraint I found that it uses the overall component mole fractions and thus the solids are incorrectly being included in those calculations. The fix will take some thought,

[lbianchi-lbl]

Merging #977 might address part of the problem, but the underlying issue would still remain.

[lbianchi-lbl]

@andrewlee94 I've reopened #1176 and will be closing this under the assumption that #1213 was supposed to be linked to this issue. Feel free to reopen it if that's not the case. I see now that #1213 addresses... both? Should both #1212 and #1176 be closed?