Added iterator functions for perimeter nodes.
Added iterator functions for perimeter nodes.

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