Require platform type in metadata preamble to data

[karlmsmith]

Due to the ambiguity of the NODC code for things such as Saildrones (Unmanned Surface Vehicles), require the platform type be given in metadata preamble in the data file.

Currently "Ship", "Mooring", and "Drifting Buoy" are automatically assigned from the NODC code if the platform type is not specified. Assigning "Ship" from ship NODC codes seems pretty unambiguous and could/should be kept; thus required only if "unknown/other vessel" (eg, 3199 for US), "mooring" (eg 3164 for US), or "drifting buoy" (eg, 32DB for US).

Probably should try to standardize the type names. Currently it is free-format and there does tend to be a variety of types specified ("boat", "yacht", "ship of opportunity", "Saildrone"). Still need to figure out an appropriate list, but I assume "Unmanned Surface Vehicle" or "USV" is the standard specification for the "Saildrone brand".