Currently sage returns a pair of rational functions when asked

E = EllipticCurve('11a1')
E.multiplication_by_n(7)

I would be better if this creates a EllipticCurveIsogeny object from E to E.

CC: @JohnCremona @williamstein @robertwb

Component: elliptic curves

Keywords: isogeny

Author: Craig Citro

Reviewer: John Cremona, Chris Wuthrich

Merged: sage-4.3.2.alpha0

Issue created by migration from https://trac.sagemath.org/ticket/7262