Label closed basins in `identifyflats`

[wkearn]

TopoToolbox V2 also labels closed basins in addition to flats and sills. If we can implement this, we can replace its identifyflats completely with the libtopotoolbox implementation. It uses imregionalmin along with some post processing, which should not be too challenging to incorporate into the main scan of identifyflats.

Note that DEMs that have been filled will not have any closed basins, so the existing property-based and snapshot tests will not be sufficient to test this implementation.

[wkearn]

Actually, I misunderstood exactly what the regional minima operator does. Following Vincent (1993), regional minima are extracted by the formula

M = -dem - reconstruct(-dem-1,-dem)

which means we need morphological reconstruction to implement this directly. Computing the morphological reconstruction might not be necessary if the only purpose is to label the regional minima, which is more like a connected components operation.