Bugs in af.approx1()

Convergence checks performed on the ArrayFire routines for interpolation seem to indicate a few mistakes:

The following plot is generated using `af.INTERP.CUBIC`

The order of convergence comes out to be , while the expected convergence rate is . I checked with `af.INTERP.CUBIC_SPLINE` as well. Even in that case the order of convergence comes out to be 


Another issue is the fact that the error doesn't drop below 1e-7. The SciPy interpolation function `interp1d` using cubic interpolation gives this:


For your reference - this is the [code](https://gist.github.com/ShyamSS-95/ad0065e984b76d1ccaecc5e21b425734) I'd used to generate the plots.

Also regarding issue #130 - I've realised that the mistake was on my part. I should have added the off-set in the indexing to the normalized interpolant points.

Start by running the linked reproduction code and examining af.approx1 with af.INTERP.CUBIC and af.INTERP.CUBIC_SPLINE. Compare the observed convergence rates and error floor with SciPy's interp1d; done means identifying and correcting the interpolation behavior so the expected convergence and accuracy are restored.

Vom Indexierungsmodell aus dem Issue-Text verfasst.