- cufinufft3d.cu
  This file contains the execution functions for 3d type1,2 that are called in ../cufinufft.cu

- spreadinterp3d.cu
  This file contains all the GPU kernels for 3d spreading, interpolation.

- interp3d_wrapper.cu
  Wrappers for 3d interpolations. Two methods are implemented:
    (1) nonuniform driven,
    (2) subproblem

- spread3d_wrapper.cu
  Wrappers for 3d spreading. Three methods are implemented:
    (1) nonuniform points driven,
    (2) subproblem,
    (4) block gather
