Calculate PSWF using an old SDE routine re-written in Python

Find Spheroidal function with M = 6, alpha = 1 using the rational approximations discussed by Fred Schwab in ‘Indirect Imaging’.

This routine was checked against Fred’s SPHFN routine, and agreed to about the 7th significant digit.

The griddata function is (1-NU**2)*GRDSF(NU) where NU is the distance to the edge. The grid correction function is just 1/GRDSF(NU) where NU is now the distance to the edge of the image.