Adding custom expressions in unit model config arguments

[zacharybinger]

Description

There is some existing IDAES functionality for handling adding custom constraints and relationships to a model when defining the model. There is some desire to have similar functionality for the RO unit models (perhaps more models) specifically for friction_factor expressions (perhaps other options too).

Motivation

It would provide a user a more direct place to add custom relationships to a unit model and it would implement them when the unit model is built rather than later on the flowsheet level.

Possible Implementation

IDAES ControlVolume1D has this feature and can likely be replicated

Additional Context

No response