Adding ParameterizedLinearRepn and corresponding walker

[emma58]

Fixes # .

Summary/Motivation:

With eventual bilevel programming applications in mind, this PR adds a ParameterizedLinearRepn object that has the same structure as LinearRepn but allows for an argument wrt to specify a list of Vars that will be treated as data for the purposes of creating the repn.

Changes proposed in this PR:

• Adds ParameterizedLinearRepn component that inherits from LinearRepn but has to re-implement everything that assumes that constants and coefficients are numbers rather than expressions.
• Adds ParameterizedLinearRepnVisitor that inherits from LinearRepnVisitor but adds a "pseudo-constant" expression type and new exitNode handlers for psuedo-constants (i.e., Pyomo expressions that we are treating as constant mathematically)
• Adds handling for nan in assertExpressionsEqual

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