Added active_faces property to ModelGrid.
Added active_faces property to ModelGrid.

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