Subsets a network to features upstream of a given outlet.
For non-dendritic networks, the function identifies flowpaths connected
to the upstream basin via fromnode/tonode that are not reachable by
upstream navigation alone. This is useful for networks where an upstream
navigation returns a basin that contains a nested basin (closed or intersecting)
connected through diversions. If only_up is FALSE, those
nested basins are captured by navigating downstream from missed diversions
to find their outlets, then navigating upstream from those outlets.
Note: If a diversion leaves a basin entirely, the subset will include the
entire basin upstream of where the diversion terminates. this function
will return both closed basins and intersecting basins.