Added docstring for number_of_faces.
Added docstring for number_of_faces.

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