added set_nodata_nodes_to_fixed_gradient() functionality
added set_nodata_nodes_to_fixed_gradient() functionality

only tested it on a toy grid. will be testing on DEM today!

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