Fixing ``zriddrp480''

The numerical recipes routine ``zriddr'' also has a small bug: For wildly varying functions such as ``func_Schrodinger'', zriddr sometimes returns too early. To fix this, replace the code fragment

if (fabs(xnew-ans) <= xacc) ... ,

in ``zriddrp480'' with

if (fabs(xl-xh) <= xacc) ... .