Adding solution pool functionality to Pyomo

[whart222]

Summary

The alternative_solutions contrib package includes a rudimentary solution pool object. A more comprehensive capability is needed for this contrib package, but maybe this should hosted in a new solution_pool contrib package.

Rationale

Solution pools are intrinsic to the functionality of the methods in pyomo.contrib.alternative_solutions. However, solution pools are also natural building blocks for a variety of techniques like branch-and-bound and optimiality-based bounds tightening. A pyomo.contrib.solution_pool package would promote this capability and ensure that it has suitable functionality to support algorithms beyond enumeration of alternative solutions.

Description

I have prototyped a solution pool methodology outside of pyomo. This is sufficiently complex that a separate contrib package makes sense. Further, I imagine that this would be a suitable target for performance optimizations (e.g. cython), since pool management is a kernel of optimization solvers.

Additional information

None.

[michaelbynum]

We have been working on this some in the solver interface refactor (see #1030, #3476, and https://pyomo.readthedocs.io/en/latest/explanation/experimental/solvers.html). There is still lots of work to do though.