routing at boundaries
routing at boundaries

We now forbid flow EVER to come into a grid from a perimeter node. This
is now 100% consistent with our definition that perimeter nodes MUST
lack cells, which it was not before. Update a couple of doctests to
reflect this, and explicitly document it.

ManuSchmi88/landlab,csherwood-usgs/landlab,ManuSchmi88/landlab,ManuSchmi88/landlab,landlab/landlab,amandersillinois/landlab,landlab/landlab,cmshobe/landlab,landlab/landlab,Carralex/landlab,RondaStrauch/landlab,SiccarPoint/landlab,Carralex/landlab,RondaStrauch/landlab,laijingtao/landlab,cmshobe/landlab,SiccarPoint/landlab,csherwood-usgs/landlab,laijingtao/landlab,cmshobe/landlab,Carralex/landlab,RondaStrauch/landlab,amandersillinois/landlab