Document what the Modular Property Framework considers to be an "Equation of State"

[dallan-keylogic]

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A repeated source of confusion for users starting out with IDAES is what the modular property framework considers to be an "equation of state". See, for example, #1613 . They think that because they're not using a classic EoS like cubic, PC-SAFT, etc., they don't need to provide anything for a phase. However, the modular property framework uses "equation of state" more broadly, in a way that includes an Ideal equation of state for gas, liquid, and solid phases as well as activity models as equations of state.

~~Presently, this information doesn't seem to be present in the documentation. Therefore, it should be added.~~

It was under the "Phases" tab. Nevertheless, since the examples are "Ideal" (gas?) and "Cubic", a new user may be confused.

Additionally, the modular properties should verify that an equation of state is provided in the config dictionary while building either the parameter or state block.