Support non-geographical data

[davidbrochart]

xarray-leaflet could be a general purpose visualization tool, not specific to geographical data. For that we could use CRS.Simple (see https://leafletjs.com/examples/crs-simple/crs-simple.html).

[davidbrochart]

I think that for this to be possible, the re-projection step should not be hard-coded, but should be treated as a general (optional) transformation.