Reduce disk space on surface datasets by using the urban lookup table

There are 34 urban fields on the surface dataset that are output as double precision at the resolution of the output grid. We could save space and make it easier to bring in updates to urban fields in, by just keeping the region ID on the surface dataset, and referencing the lookup table.

Roughly 25% of the surface dataset is made up of such urban fields. We could save about 50GBytes per new set of surface datasets made. The entire raw dataset (with region ID at 0.05x0.05 degree resolution) is only about one GByte.

@olyson @dlawrenncar