added d4 functionality to has_boundary_neighbor()
added d4 functionality to has_boundary_neighbor()

added method keyword to has_boundary_neighbor to allow to test boundary
neighbors using d8 (the default) or d4. The d4 boundary neighbor method
is needed for link boundary condition handling

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