.ugrid.sel(x=..., y=...) should return a 2D result

[Huite]

The docstring of sel currently mentions that it will do orthogonal indexing, but it gives a somewhat confusing result:

• It's 1D
• Its primary dimension is the UGRID dimension
• It has silently dropped locations that are outside of the grid

It would make more sense to just rely on the same logic as rasterize here.