BUG/ENH: modernize cosmic_gps methods

[jklenzing]

The current structure of the cosmic_gps instrument needs to be updated for the pysat 3.0.0 release. While working on bugs related to pysat/pysat#337, a number of areas to improve have been noted, including

• Multiple spacecraft and antennas lead to possible "duplicate" timestamps. The enforcement of unique times on data requires the addition of fractional seconds to the data timestamps. While this does not affect some analysis methods (eg, climatology), a better long-term solution for this is to move to xarray.
• This also affects the file lists. The current system solves this by adding fractional seconds (<0.01s in total) to timestamps both for the file lists. Because "duplicate" files are effectively allowed, a user could potentially download multiple versions of a data file over the lifetime of the mission and wind up loading all data sets, including old versions of the same file.
• The download method and storing using the 'YYYY.DDD' format makes the parsing less clean. There's currently a workaround in BUG: recognize string variables in parse_delimited_files pysat#439, but this could be cleaned up with a breaking change.

Proposed updates:

• Handle the data using xarray
• Expand the fname string to include version numbers and potentially other keywords. May require updates in the core instrument object (breaking)
• Improve the directory structure by letting pysat handle the day subdirectories (breaking)

ref: https://cdaac-www.cosmic.ucar.edu/cdaac/cgi_bin/fileFormats.cgi?type=ionPrf

[jklenzing]

A possible route forward for when pysatCDAAC is created:

• Add another dataset (GRACE?) that does not require username / password
• generalize download / list_files, etc so that these routines can be tested

EDIT: created #3 for discussion of generalized method functions.

[aburrell]

GRACE is a useful data set, I think this is a good idea.

[rstoneback]

When I just checked on CDAAC I got prompted for a username and password for GRACE data. I’m all for finding a way to get the files tested. We could potentially store credentials on travis CI as environment variables.

[aburrell]

That's a bad idea. I'd hate for pysat to get a bad rep because CDAAC got swamped after someone stole our account details. Unless there's a way to do it silently.

[rstoneback]

Download method has been updated for the public option and the subdirectories were updated to year/day in #17.

[rstoneback]

Version information added to the COSMIC file format string in #17.

[rstoneback]

xarray support added in #18.

[rstoneback]

Version information added to the COSMIC file format string in #17.

The code over in #17 has been improved since the first note. Now, time mangling depends entirely upon invariant parsed information from filenames, so newer versions of a file will get the same time offset and be recognized as a newer version.