parmest enhancement: support params or variables

[adowling2]

Summary

As part of the IDAES end-to-end uncertainty discussion, it came to light that:

• Parameter estimation requires uncertainty model parameters to be defined as Pyomo variables
• Robust optimization (PyROS) requires uncertain model parameters to be defined as Pyomo parameters (mutable)

Recommendation/Feature Request:

• Generalize parmest to support either params or variables for uncertainty parameters that need to be regressed
• PyROS already has some code to handle this conversion under the hood
• Long-term: develop general utility code to support this conversion under the hood, we can use such a utility in parmest, PyROS, and the entire end-to-end uncertainty framework

Rationale

The distinction between variables and parameters becomes confusing to users, especially new users.

[adowling2]

@jsiirola @kaklise This came up in a PyROS development call today. This also touches on IDAES UQ work.

[adowling2]

Look for the expression replacement visitor in PyROS.

[adowling2]

Leading idea: extract the conversion code from PyROS, make it into a general Pyomo utility is use across the uncertainty ecosystem