Added diagonals_at_node property.
Added diagonals_at_node property.

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