Expose weights as a "normal" DataArray

[huard]

The current weight matrix is flattened, with dimensions (out_dim, in_dim). I think it would be useful, including for debugging, to reshape the weights so they're exposed as lon_out, lat_out, lon_in, lat_in.

We could alternatively use the same dimension names as the input and output grids, but if they're both called lat and lon, this won't work.