Structure factor of the multilayer. for DWBA use.
- corr_qxy.c is for the xy averaged, R*besselj(qR)
- corr_qy.c is for the y averaged, cos(qx*y)

corr(double qx, double qz1, double qz2, double sig1, double sig2, double zint1, double zint2, double hcor, double vcor, double H, double *y)