PEP: Standardize Example Imports

[mrmundt]

Fixes #1026

Summary/Motivation:

As promised, I was frustrated with something solver-refactor related yesterday, so I took break to resolve this PEP. This standardizes all files in the examples directory to use the same import standard (import pyomo.environ as pyo).

Changes proposed in this PR:

• Standardize imports to import pyomo.environ as pyo
• Fix other import * statements as appropriate

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[blnicho]

I found lots of places where pyo. was added by mistake.