Added methods to return the diagonal cells of a given cell ID to ModelGrid.
Added methods to return the diagonal cells of a given cell ID to ModelGrid.

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