add _links_and_nbrs_at_node to lake_mapper, with doctest
add _links_and_nbrs_at_node to lake_mapper, with doctest

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