ecpp_class_poly_genus

Sets F to the factor of the Hilbert class polynomial $H_D$ modulo $n$ (the modulus of ctx) that corresponds to the principal genus. Here $D = p_1^* \cdots p_g^*$ with the $p_i^*$ given in pstar (the signed odd primes $p^* = (-1)^{(p-1)/2} p$ dividing $D$ and, for even $D$, the factor $q_0 \in \{-4, 8, -8\}$), and sqrts[i] is a square root of pstar[i] modulo $n$, which must exist. Over the genus field $K(\sqrt{p_1^*}, \ldots, \sqrt{p_g^*})$, $H_D$ factors into $2^{g-1}$ conjugate factors of degree $h(D)/2^{g-1}$, whose coefficients are identified numerically from the values of $j$ at the reduced forms grouped by genus; F is the image of one of them modulo $n$, so that finding a root costs a fraction $2^{-(g-1)}$ of a root of $H_D$. Returns 1 on success and 0 if $g < 2$ or the coefficients could not be identified. Requires $D$ fundamental.