Function: ellisisom
Section: elliptic_curves
C-Name: ellisisom
Prototype: GG
Help: ellisisom(E,F): return 0 if the elliptic curves E and F defined over the
 same number field are not isomorphic, otherwise return [u,r,s,t] suitable for
 ellchangecurve, mapping E to F.
Doc:
 return $0$ if the elliptic curves $E$ and $F$ defined over the same number
 field are not isomorphic, otherwise return \kbd{[u,r,s,t]} suitable for
 \kbd{ellchangecurve}, mapping $E$ to $F$.
