MixedIntegerLinearProgram.add_constraint: Option to return row indices, fix handling of empty constraints #34878
Description
(from #21003 comment:136)
... for use with methods that take row indices as input, such as row, row_bounds, remove_constraint.
If a backend does not map a constraint to a unique new row (or check_redundant=True and the constraint is discarded by the frontend), the method may return None.
We also fix a longstanding bug in how empty constraints are handled in add_constraint.
The changed interface (an optional return value) is backward-compatible with user code.
We also add a Feature CVXOPT so that the backend gets tested again.
Component: linear programming
Author: Matthias Koeppe, Martin Rubey
Branch/Commit: 3b929f8
Reviewer: Martin Rubey, Matthias Koeppe
Issue created by migration from https://trac.sagemath.org/ticket/34878