QuadraticForm and BinaryQF objects have a .polynomial() method to return the quadratic form represented as a multivariate polynomial, but the converse is not currently available. This patch adds it.

Component: quadratic forms

Author: Lorenz Panny

Branch/Commit: 9224d7e

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/34863