BUG/ENH: Added support for remote file list

[rstoneback]

COSMIC currently lacks support for returning a list of online files. Add it so that gps.download_updated_files() works.

[jklenzing]

Linking to pysat/pysat#1012. We could test by using the current routine from pysatNASA in a test branch of pysatCDAAC, then port all those files to the core code for usage here.