radon: Use numpy.interp/scipy.interpolate.

Scipy supports all interpolation kinds (nearest, linear) we need, while
numpy supports only linear interpolation. The numpy interpolation is
substantially faster, so this is used even though it complicates the
code slightly.

radon: use np.interp for linear interpolation.

radon: use scipy.interpolate for interpolation.

radon: Use scipy.interpolate for interpolation.

numpy for linear, scipy for other interpolations.