Support loading SECS data from CDAWeb

[jameswilburlewis]

We recently had a long outage on vmo.igpp.ucla.edu, which hosts the SECS and EICS data sets. James Weigand says that this data is also available at CDAWeb, so we should add support for that with an spdf keyword.

[nickssl]

Where is the SECS data located? It is not in the main SPDF data directory:

https://spdf.gsfc.nasa.gov/pub/data/

Also, SECS is not in the list of missions of the CDAWeb GUI.

[jameswilburlewis]

I think it's here:

https://spdf.gsfc.nasa.gov/pub/data/aaa_special-purpose-datasets/spherical-elementary-and-equivalent-ionospheric-currents-weygand/

[nickssl]

Done.