Deleted functions for nearest neighbor polar/inverse polar transform

Because the bilinear interpolation polar/inverse polar transformation functions work much better, and don't seem noticably slower, I deleted the nearest neighbor polar/inverse polar functions, as they're not needed.

I also re-named the faster "rolling" azimuthal mean filter from "azi_mean_filter_2" to "azi_mean_filter_fast", so that it's a bit clearer what it actually is.