created set_fixed_link_boundaries_at_grid_edges
created set_fixed_link_boundaries_at_grid_edges

this handles link boundary conditions AND fixed gradient nodes along
the edge of a raster.

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