Create specialised raster calc_slope_at_node
Create specialised raster calc_slope_at_node

Create equivalent method in raster. This one breaks the raster square
patches into pairs of triangles under the hood, removing potential
preference for one direction on the raster in the slopes. Note this may
break quite badly with the graph merge.

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