Make use of SciPy's RegularGridInterpolator for faster regridding.

First pass at implementing way faster interpolation for rectilinear regridding. In one case grid size (800, 1200) I've seen execution time go from ~31m down to ~1s.

This has been a tricky PR to pull together due to the high number of value changes. I've done a reasonable investigation on the npy files, but it is by no means comprehensive - I'd be willing to investigate further is needs be.

Don't merge just yet, but I would like to get some eyes on this as it stands.