Add more interpolation options

[jbednar]

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 or 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 that can be supported, which also involve interpolation or approximating the underlying surface.