Add support for rioxarray rasterio and update tests

[hoxbro]

Fixes #929

[ianthomas23]

I think the use of data.rio.crs is preferred as it checks a number of possible locations in the xarray for the CRS. It does however return an instance of rasterio.crs.CRS which we cannot pass to cartopy directly, so here I am converting to WKT first. This may not be ideal as it might give us various warnings, let's see what CI gives us.

An alternative to the to_wkt() is to add specific support in our CRS to cartopy for a rasterio.crs.CRS.

[maximlt]

Great! Thanks a lot @hoxbro !