Function: ellsigma
Section: elliptic_curves
C-Name: ellsigma
Prototype: GGD0,L,p
Help: ellsigma(E,z,{flag=0}): E being given by ellinit, returns the 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.
Doc:
 $E$ being given by \kbd{ellinit},
 returns the value at $z$ of the Weierstrass $\sigma$ function of the period
 lattice $L$ of $E$:
 $$ \sigma(z, L) = z \prod_{\omega\in L^*} \left(1 -
 \dfrac{z}{\omega}\right)e^{\dfrac{z}{\omega} + \dfrac{z^2}{2\omega^2}}$$
 Alternatively, one can input a lattice basis $[\omega_1,\omega_2]$ directly
 instead of $E$.

 If $\fl=1$, computes an (arbitrary) determination of $\log(\sigma(z))$.

 If $\fl=2,3$, same using the product expansion instead of theta series.
