Subsetting to a single point

[mpiannucci]

Pretty useful to slice to a given point, a little tricky to figure out how this should work with ugrid, but would be very useful and it is in fact a subset in space

[ChrisBarker-NOAA]

hmm -- this really strikes me a different use case, but I think a UGRID with a single face is legal -- so we might be able to make that work, and indeed, it might work almost out of the box:

what happens when you subset to a size_zero bounding box?

if that fails, maybe a size-eps bounding box?