Removed _calculate_flux_divergence_at_active_cells_slow.
Removed _calculate_flux_divergence_at_active_cells_slow.

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