need standard way to get lists of observed variables, platforms, etc

[gaelforget]

Here are such methods currently available

NOAA.list_stations()
NOAA.list_realtime()
GDP.list_files()
ArgoFiles.list_floats()
OceanSites.index()
OceanOPS.get_list()
OceanOPS.list_platform_types()

missing

• ShipCruise / CCHDO (see data sets / meta data to add? #107)
• Gliders
• CloudDrift

design

• public API should be something like list(NOAAbuoy()) to get the list of buoys (platforms) and then list(NOAAbuoy(),41044) to get the list of variables
• or maybe this should be called scan, listing, or something else?
• struct could be introduce to standardize results (e.g., ListOfPlatforms, ListOfVariables`, ...)
• units, long names, and other meta data should be listed / included as well as much as possible