Medium: [Thermo] Centralize activity-model selector validation

[Mazhar331]

Follow-up from #120 / #56

PR #120 fixes #56 by giving DynamicExtractor a default equilibrium function and by adding local validation for the documented activity-model selectors in extraction constructors. That PR intentionally keeps the broader selector source-of-truth cleanup out of scope so the #56 bug fix stays narrow.

Problem

PharmaPy/Extractors.py now has VALID_GAMMA_METHODS = ('ideal', 'UNIFAC', 'UNIQUAC'), while PharmaPy/Phases.py still encodes the same selector set implicitly inside getActivityCoeff. If a supported activity-coefficient model is added, renamed, or deprecated in one place, the extractor validation can drift from the thermo implementation.

The same family of selectors appears under several names, including gamma_model, gamma_method, and activity_model. Related open issues #27 and #59 show adjacent places where activity-model routing already gets dropped or misapplied.

Scope

• Define one shared source of truth for valid activity-coefficient model selectors, preferably near the thermo implementation that consumes them.
• Reuse that source for extraction validation added in Fix DynamicExtractor default equilibrium function #120.
• Audit public producers and consumers of gamma_model, gamma_method, and activity_model in PharmaPy/Phases.py, PharmaPy/Extractors.py, PharmaPy/DynamicExtraction.py, PharmaPy/Distillation.py, and PharmaPy/Evaporators.py.
• Decide whether selector naming can be unified safely; if not, preserve public aliases and document the compatibility boundary.
• Add focused tests for invalid selectors at public boundaries and at least one caller-to-callee handoff where practical.
• Include a units/docs/comments audit. The selectors themselves are categorical, while activity coefficients and distribution coefficients remain dimensionless [-].

References

• Deferral source: Fix DynamicExtractor default equilibrium function #120
• Original bug fix scope: High: [DynamicExtraction] DynamicExtractor default constructor leaves k_fun undefined #56
• Related thermo-routing defects: High: [Evaporators] Activity model is passed positionally and VLE falls back to ideal gamma #27, Medium: [Distillation] VLE calls drop gamma_model and use ideal activity coefficients #59