Documention and Test Standardized Duals Sign Convention

[mrmundt]

Summary

We will have a standard duals sign convention, choosing Gurobi. We need to document and test this.

Other related issues: #1957, #2831

Rationale

We are taking a stand and making a choice, whoo-hoo.

Description

We have decided that we are going to use the same sign convention for duals as Gurobi and need to ensure that we (1) are actually doing that and (2) document it.

Additional information