Quality-of-life: automatically add top and bottom as coordinate to MF6 output

[Huite]

After discussion with @FransRoelofsen: making e.g. a streamfunction plot or a cross-section plot requires manually adding top and bottom coordinates to the DataArray -- this while we provide the GRB file during reading to infer the grid topology.

It seems like a minor effort to add top and bottom data by default: it's been read from the GRB file anyway, and we require the GRB file to read budget and head results.