The MIP backends just need a method base_ring(), say, that returns the base ring that the backend wants.

Apply

• attachment: trac_13650_base_ring.patch
• attachment: trac_13650_normalize_coefficients.patch

Depends on #13646
Depends on #12533

CC: @dimpase @nathanncohen

Component: linear programming

Author: Volker Braun, Dmitrii Pasechnik

Reviewer: Dmitrii Pasechnik, Volker Braun

Merged: sage-5.5.rc0

Issue created by migration from https://trac.sagemath.org/ticket/13650