try to improve accuracy of wscreen formula

nifty_gridder.cc

@@ -594,8 +594,11 @@ template<typename T> class GridderConfig
     complex<T> wscreen(double x, double y, double w, bool adjoint) const
       {
       constexpr double pi = 3.141592653589793238462643383279502884197;
-      double n = cos(sqrt(x+y)), xn = 1./n;
-      double phase = 2*pi*w*(n-1);
+      double eps = sqrt(x+y);
+      double s = sin(eps);
+      double nm1 = -s*s/(1.+cos(eps));
+      double n = nm1+1., xn = 1./n;
+      double phase = 2*pi*w*nm1;
       if (adjoint) phase *= -1;
       return complex<T>(cos(phase)*xn, sin(phase)*xn);
       }