Graphics: Write hotspot maps at NetCDF
The GeoTiffs we're producing have a single band with discrete values (0-19), each of which maps to a set of RGB values and a corresponding category (like "Surplus, 10-20 yr"). We need to produce a single-variable NetCDF that uses numeric codes like these, and supply a legend (or perhaps a coordinate variable) that identifies each of these codes. (Although we can generate a NetCDF like this by running gdal_translate on the GeoTiffs, the integer to RGB mapping is not always consistent across files, so we'd need to write something custom.) This version of the data should include a table that translates the integer codes into labels and provides suggested RGB values in addition to the raster.
Also, we should provide a three-variable NetCDF in which the raw values of the composite indicators (surplus, deficit, both) are stored as three separate variables. It would be up to the program that's consuming these files to bin and stylize the data appropriately.
Requested by @mwfarthing.