Added more doctests for number_of_core_nodes/cells.
Added more doctests for number_of_core_nodes/cells.

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