Add more interpolation options

As of version 0.6, Datashader supports nearest-neighbor and bilinear interpolation for regridding rasters and rasterizing trimesh grids. There are many other types of interpolation that can more accurately approximate the underlying continuous surface, such as [bicubic interpolation](https://en.wikipedia.org/wiki/Bicubic_interpolation) or
[Kriging](https://en.wikipedia.org/wiki/Kriging), and it would be great if Datashader supported these.

If we get more ambitious, there are also various types of [regridding that involve reprojection](https://climatedataguide.ucar.edu/climate-data-tools-and-analysis/regridding-overview) that can be supported, which also involve interpolation or approximating the underlying surface.