Added doctest for number_of_nodes.
Added doctest for number_of_nodes.

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