enable specific horizontal/vertical mappers for raster
enable specific horizontal/vertical mappers for raster

Needs to come for voronoi too, but needs a bit more work from Greg on
the ability to resolve values on the links onto cartesian coords first,
I think.

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