add plot.colors.py for Landlab-themed colormaps
add plot.colors.py for Landlab-themed colormaps

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