Added link_dirs_at_node function.
Added link_dirs_at_node function.

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