Function: ellfromj
Section: elliptic_curves
C-Name: ellfromj
Prototype: G
Help: ellfromj(j): returns the coefficients [a1,a2,a3,a4,a6] of a fixed
 elliptic curve with j-invariant j.
Doc: returns the coefficients $[a_1,a_2,a_3,a_4,a_6]$ of a fixed elliptic curve
 with $j$-invariant $j$.
