Added module-level documentation.
Added module-level documentation.

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