create pandasindex automatically where appropriate

[dcherian]

Currently for anything but slicing, we drop the index.
Tracking this TODO:

https://github.com/dcherian/rasterix/blob/54131432a43a872498e97395a3b39ed2d73ee752/src/rasterix/raster_index.py#L273-L283

[benbovy]

Created pydata/xarray#10477.