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.