Add substantial filtering to NED fetch to only retrieve US files.
Add substantial filtering to NED fetch to only retrieve US files.

Also tries to avoid fetching duplicate tiles -- only retrieves
more recent tile versions.

Wireless-Innovation-Forum/Spectrum-Access-System,Wireless-Innovation-Forum/Spectrum-Access-System,gregbillock/Spectrum-Access-System,krlinus/Spectrum-Access-System,krlinus/Spectrum-Access-System,gregbillock/Spectrum-Access-System,Wireless-Innovation-Forum/Spectrum-Access-System,gregbillock/Spectrum-Access-System,Wireless-Innovation-Forum/Spectrum-Access-System,krlinus/Spectrum-Access-System,krlinus/Spectrum-Access-System,gregbillock/Spectrum-Access-System