A faster 3D interpolation to replace scipy.interpolate.RegularGridInterpolator()

Implemented after https://stackoverflow.com/questions/41220617/python-3d-interpolation-speedup.

Installation (requires cython):

python3 setup.py install

Usage:

from interp3d import interp_3d
import numpy as np

x = np.linspace(0,2.5,100)
y = np.linspace(-1,.5,50)
z = np.linspace(5,25,125)

X,Y,Z = np.meshgrid(x,y,z,indexing='ij')
arr = X+2*Y-3*Z

interp = interp_3d.Interp3D(arr, x,y,z)

from scipy.interpolate import RegularGridInterpolator
interp_si = RegularGridInterpolator((x,y,z),arr)

x0, y0, z0 = (1.1,0.25, 7.5)
print('this class {}'.format(interp((x0,y0,z0))))
print('scipy.interpolate.RegularGridInterpolator() {}'.format(interp_si((x0,y0,z0)), x0+2*y0-3*z0))
print('exact {}'.format(x0+2*y0-3*z0))