WV-1057: Colormap support for MODIS Collection 6

For MODIS Collection 6, the following are needed:

* Ability to have multiple color maps, see http://map2.vis.earthdata.nasa.gov/colormaps/new/MODIS_Aqua_Cloud_Effective_Radius_v6.xml
* Specify a legend if the colormap is using coloring tricks to preserve a continuous spectrum for blocked regions (AOD)