Diagnostics tool for ill-posed constraints

[andrewlee94]

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Waiting on https://github.com/Pyomo/pyomo/pull/3376

Summary/Motivation:

As part of working on the new scaling tools, I realised there are some simple checks we can do for detecting poorly-posed constraints that could cause scaling issues. This PR adds a new expression walker that looks for the following signs of poor scaling in constraints:

• sum expressions with terms which have significant differences in magnitude.
• sum expressions where terms cancel out (with a catch for cases of constant == sum()).

Changes proposed in this PR:

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