associate data from one topology aspect to another

[veenstrajelmer]

It would be a nice addition to "interpolate" e.g. edge variables to faces, or nodes to edges, or any other shift in topology aspect. Currently, there is a node/edge-to-face interpolation function in dfm_tools, but this is a bit too specific and it would be great to land this in xugrid instead.

It would be nice if the user can do things like:

uda = uds[edge_var]
uda.to_faces()

uda = uds[node_var]
uda.to_edges()

[Huite]

Duplicate of: #173

(This was first, but I wrote more in the other one...)