Neatened up route_flow_dn
Neatened up route_flow_dn

* Doctests should now pass
* Improved field handling

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