LinearStandardFormCompiler: allow_nonlinear and ignore_ctypes options

[bknueven]

Fixes # N/A

Summary/Motivation:

LinearStandardFormCompiler is a great tool for rapidly expanding a Pyomo model into the form need for (mixed-integer) linear programming solvers. Many of these solvers also handle quadratic constraints, SOS constraints, quadratic objectives, and even sometimes general nonlinear constraints and objectives. This PR would augment the LinearStandardFormCompiler to handle functional forms it cannot compile directly are either passed on or ignored and do not raise an error (new option allow_nonlinear and extra_valid_ctypes).

Changes proposed in this PR:

• Add option for LinearStandardFormCompiler to return non-linear constraints / objectives it encounters instead of raising an error (option allow_nonlinear).
• Add option for additional valid ctypes, to be handled by the caller. Useful for SOS constraints. (option ignore_ctypes)

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