Added more documentation for ModelDataFieldsMixIn.
Added more documentation for ModelDataFieldsMixIn.

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