Function: ellisoncurve
Section: elliptic_curves
C-Name: ellisoncurve
Prototype: GG
Help: ellisoncurve(E,z): true(1) if z is on elliptic curve E, false(0) if not.
Doc: gives 1 (i.e.~true) if the point $z$ is on the elliptic curve $E$, 0
 otherwise. If $E$ or $z$ have imprecise coefficients, an attempt is made to
 take this into account, i.e.~an imprecise equality is checked, not a precise
 one. It is allowed for $z$ to be a vector of points in which case a vector
 (of the same type) is returned.
Variant: Also available is \fun{int}{oncurve}{GEN E, GEN z} which does not
 accept vectors of points.
