2D coordinates: convert RasterIndex to TreeIndex

[benbovy]

For small-ish 2D x/y coordinates (affine transformation with rotation and/or shear) it might be nice to provide a convenient function or accessor method to convert the RasterIndex into a TreeIndex (pydata/xarray#10478) at some point. I guess that the latter would be more useful than a wrapped CoordinateTransformIndex for a few common applications? Especially if TreeIndex eventually supports lat/lon points (possibly via xoak).