Alignment

[benbovy]

Because a DGGSIndex wraps a PandasIndex, we can (should?) support alignment of Dataset / DataArray objects just like with Xarray default (pandas) indexes, here based on cell ids.

At the very least, we should raise an error during alignment if the grid type and parameters of the objects to align don't match exactly.

Later, I guess it will be possible to support fancier, grid-aware alignment like, e.g., align two objects on the same grid but at two different resolutions. That said, I'm not sure that it is a good idea to do that in an automated way.