DOC: add more examples

[aburrell]

Improve documentation by adding examples for:

• plotting VTEC (re-creating gnss_tec_vtec_example.png)
• using methods.dmsp.smooth_ram_drifts
• using methods.dmsp.update_DMSP_ephemeris
• using methods.dmsp.add_drift_unit_vectors
• using methods.dmsp.add_drifts_polar_cap_x_y
• using methods.jro.calc_measurement_loc

[aburrell]

@rstoneback these examples are from your code, but more importantly require the use of the DMSP ephem Instrument, which we don't have in any public space. Which repository should it go in? I think the lack of a download function was an issue, perhaps it can be solved?

[rstoneback]

The DMSP functions were written for the DINEOFs SuperDARN DMSP thing. I’m not sure where they should go. Don’t really have to go anywhere. I got the DMSP ephem from Rob Redmon but it may be possible the data is more public now, given how some funding sources have shifted to more public disclosure. I did find that updated ephemerides are in the SSJ and SSM datasets. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016JA023339 CDAWeb has coverage from 2009 - jan 1 2015 via SSM.

[aburrell]

Would be worth contacting him to see if they are available. It would certainly be useful to update the DMSP IVM ephemera.

I think the routines are useful for many DMSP IVM analysis processes and it's ok to have them here. But, obviously the results are better with updated locations 🐬

[rstoneback]

update_DMSP_ephemeris still pending.

[aburrell]

Moving this to "future", since it's a long road to get the ephemeris instrument.