[PEP] Process for handling/testing examples

[blnicho]

Summary

Pyomo examples are inconsistent in how they are implemented and tested and where they are located. We should come up with, implement, and document a process for how examples should be handled within repositories under the Pyomo GitHub organization.

Rationale

A coherent and standardized process will help us keep existing examples organized and maintained and will help future developers who wish to contribute examples.

Relevant comment from @bernalde from #2932:

Where should we draw the line between a set of examples, tests, and supported models? In particular, for GDP, we have GDPLib https://github.com/SECQUOIA/gdplib, which could host these two examples. What's the motivation for adding examples to the main Pyomo repository?

Description

Related issues/tasks:

• Rework examples directory  #101
• [PEP] Standardizing Pyomo Imports in examples as "import pyomo.environ as pyo"  #1026
• Fix non-doctest documentation tests #1950