Added a couple doctests for links_at_node.
Added a couple doctests for links_at_node.

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