Draft: Hycom surface only
When downloading ocean data using the HYCOM module and user specifies bottom = 0
or top = bottom = None
, fetch request should be made with binning of dz = 0
or dz = 1
(not sure which), so that only surface layer values are downloaded. This will create two different sets of hashes in the index module for HYCOM requests, one for surface-only requests, and one for all-water-depths requests.