code by will grissom that implements the clever nufft method
from the following paper
@a greengard:04:atn siam-rev 46 3 443-54 . 2004
Leslie Greengard June-Yub Lee
Accelerating the nonuniform fast Fourier transform
@u doi 10.1137/S003614450343200X
if you want to run this on a PC, you will need to use the matlab "mex"
command to compile the back_grid.c and forw_grid.c routines into mex
executables for the PC. i don't know the specifics of how to do this
on a PC but "help mex" should explain.