chunked version of the methods

[keewis]

With #151 we're now able to support a lazy cell_ids coordinate, but the methods (cell_centers, cell_boundaries, zoom_to from #62) will still compute.

Most interactions with the cell ids are element-wise and thus embarrassingly parallel, so it should be very easy to change these operations to support chunked arrays (possibly through xr.apply_ufunc?)

This issue serves as a reminder for me (or anyone else) to look into this.