Deprecate pyomo.core.plugins.transform.model.to_standard_form()

[jsiirola]

Fixes # .

Summary/Motivation:

This deprecates the pyomo.core.plugins.transform.model.to_standard_form() (and implicitly the entire pyomo.core.plugins.transform.model module). This functionality is not tested at all, and has been replaced by the WriterFactory('compile_standard_form') writer.

Changes proposed in this PR:

• Deprecate pyomo.core.plugins.transform.model.to_standard_form()

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[quantresearch1]

Nice one