platform, instrument, and location keywords

[pacioos]

@amilan17 This update looks for the following keywords and their vocabularies in global attributes and adds them to the ncISO output:

platform
platform_vocabulary
instrument
instrument_vocabulary
locations
locations_vocabulary

Note that the platform, platform_vocabulary, instrument, and instrument_vocabulary attributes are already "suggested" global attributes in the ACDD-1.3 convention. I have proposed adding locations and locations_vocabulary into the next ACDD version with favorable initial response from ESIP. Note that I previously modified ncISO to look for location keywords in THREDDSMetadata. This update continues to look there but gives precedence to the global attributes if present, especially because locations_vocabulary is not captured in THREDDSMetadata. As with the existing "keywords" attribute, I look for either comma-separated or space-separated values.

Below are examples of using these fields. Note that ">" symbols would be encoded as ">":

platform: "In Situ Ocean-based Platforms > Buoys, In Situ Ocean-based Platforms > Moorings"
platform_vocabulary: "GCMD Platform Keywords"

instrument: "In Situ/Laboratory Instruments > Chemical Meters/Analyzers > > > Fluorometers, In Situ/Laboratory Instruments > Chemical Meters/Analyzers > > > Oxygen Meters, In Situ/Laboratory Instruments > Conductivity Sensors, In Situ/Laboratory Instruments > Photon/Optical Detectors > > > Turbidity Meters, In Situ/Laboratory Instruments > Profilers/Sounders > > > CTD, In Situ/Laboratory Instruments > Temperature/Humidity Sensors > > > Temperature Sensors"
instrument_vocabulary: "GCMD Instrument Keywords"

locations: "Continent > North America > United States Of America > Hawaii, Ocean > Pacific Ocean > Central Pacific Ocean > Hawaiian Islands > Oahu > Ala Wai Canal, Ocean > Pacific Ocean > Central Pacific Ocean > Hawaiian Islands > Oahu > Honolulu"
locations_vocabulary: "GCMD Location Keywords"

Each category creates its own "gmd:descriptiveKeywords" group in the ncISO output with separate "gmd:keyword" values and a "gmd:thesaurusName". Platform and instruments, like the "keywords" attribute, get assigned a "gmd:MD_KeywordTypeCode" of "theme" while locations are assigned "place".

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