Enable diffusion using link vals
Enable diffusion using link vals

LinearDiffuser can now optionally take diffusion values defined at
links. This permits second order precision calcs of diffusion.

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