ENH: JPL GPS data products

[jklenzing]

Describe the solution you'd like Support for JPL GPS data products

• [x] TEC
• [x] ROTI

Describe alternatives you've considered none

Additional context Requires #92 first

[jklenzing]

Looking over the data products, if we follow the standard set by jpl_roti, then we would have

• igs_tec wtih inst_id of '1hr', '2hr', '15min'
• lanl_vtec

Or we could try to combine in a single instrument module, but I'm not sure what that would look like yet.

https://cdaweb.gsfc.nasa.gov/pub/data/gps/

[jklenzing]

Looking at how the CDAWeb data is organized, as well as our init methods (see #121), I think the cleanest way of moving forward is platform = 'igs' name = 'gps' tags = ['tec', 'roti'] inst_ids = cadence

The JPL data is derived from the IGS database