Pyomo.DoE: Allow user to define experiment cost to include in the objective

[adowling2]

Fixes # .

Summary/Motivation:

• For some MBDoE problems (especially dynamic with many degrees of freedom), the FIM-based objective can be "flat", i.e., there are many different experiment designs with very similar information content
• Thus, it is desirable for the user to be able to specify a second objective, e.g., experiment_cost, that can also be minimized

Changes proposed in this PR:

• Extended interface to consider experiment_cost
• Allow user to specify weight for experiment cost component of the objective

TODO Before Converting from Draft:

• Decide if this is a good approach, or should the experiment_cost be specified with suffixes?
• Do we want just one auxiliary objective (experiment cost) or to support many auxiliary objectives?
• Update documentation
• Create an example
• Add tests
• Add error checking
• Run black

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[adowling2]

@adowling2 @blnicho @jsiirola Quick feedback on this concept is welcome

[djlaky]

Use the average over the scenarios instead of only the base scenario? Central finite difference in mind with this idea.

[mrmundt]

Please make sure to run black -S -C with the most recent version of black on your code.