[FEATURE] - Zone budgets for unstructured data

[Huite]

We currently have no convenient way to report zone water balances for DISV (or DISU, for that matter).

With xugrid, it's easy enough to "paint" zones, then do a groupby for boundary conditions, but this isn't the case for face flows.
With face flows, there is one additional intermediate level: you paint the face, but the flow is edge associated (and probably has a sign / direction as well). The best option is likely to defer to xugrid. I've opened Deltares/xugrid#418 to this end.

For xugrid, I'd like to abstract it such that it works for Ugrid1d as well. That would make it useful to e.g. SFR as well.