Function: ellsigma
Section: elliptic_curves
C-Name: ellsigma
Prototype: GGD0,L,p
Help: ellsigma(om,z,{flag=0}): om=[om1,om2], value of the Weierstrass sigma
 function of the lattice generated by om at z if flag = 0 (default). If flag
 = 1, arbitrary determination of the logarithm of sigma. If flag = 2 or 3,
 same but using the product expansion instead of theta series

