Add Xarray accessor `dem`

[rhugonnet]

Big task here also! Mirroring https://github.com/GlacioHack/geoutils/issues/383.

Looks like the accessor name dem is available! :smile: See discussions started in: https://github.com/pydata/xarray/issues/8040, https://github.com/corteva/rioxarray/issues/687 and https://github.com/makepath/xarray-spatial/issues/790.

Important to coordinate with xarray-spatial to avoid too much redundancy.