Function: genus2igusa
Section: elliptic_curves
C-Name: genus2igusa
Prototype: GD0,L,
Help: genus2igusa(PQ,{k}): let PQ be a polynomial P, resp. a vector [P,Q] of
 polynomials. Return the Igusa invariants of the hyperelliptic curve C/Q of
 genus 2 defined by y^2 = P, resp. y^2 + Q*y = P.
Doc: Let $PQ$ be a polynomial $P$, resp. a vector $[P,Q]$ of polynomials.
 Return the Igusa invariants $[J_{2},J_{4},J_{6},J_{8},J_{10}]$ of the
 hyperelliptic curve $C/\Q$, defined by the
 equation $y^{2} = P(x)$, resp.  $y^{2} + Q(x)*y = P(x)$.
 If $k$ is given, only return the invariant of degree $k$
 ($k$ must be even between $2$ and $10$).
