add link_at_face method/property to base.py
add link_at_face method/property to base.py

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