Repair bug in mg.forced_cell_areas
Repair bug in mg.forced_cell_areas

This was broke for a Voronoi.

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