Full implementation of thermo as an optional dependency

[yoelcortes]

Thermosteam Thermo, Mixture, and Chemicals objects may wrap around objects from the thermo library to provide support for some of their exciting features and not break support for native thermosteam objects.

This issue is meant to track progress on fully implementing the thermo library as a optional dependency.

~Additionally, it would be interesting to contribute back to thermo features regarding self contained methods/models for easier implementation of pure chemical properties (in particular user-models and model management in general) and slowly depend more and more on thermo... However, the first step is to ensure full compatibility with thermo to begin to move dependencies around.~

thermo already has well-tested features for user-defined models and model management. The best way to ensure compatibility with thermo may be to implement methods to convert thermosteam model handles to the appropriate counterparts in thermo (and possibly vice-versa while we're at it).

[yoelcortes]

This may not present any major benefits, especially considering that only the ChemicalConstantsPackage and PropertyCorrelationsPackage are used in thermo's flash implementation, which is of critical interest to thermosteam... closing this issue to prioritize the implementation of thermo's flash capabilities