Only show horizon rings for selected sondes.

[darksidelemm]

Enabling horizon rings results in a lot of clutter in areas with high sonde density:

It would probably be better if the horizon rings were only shown for sondes that have been selected, in the same way that the sonde track is only shown for selected sondes.

[darksidelemm]

The same mechanism that is used for the LOS paths (drawLOSPaths) could be used here. LOS paths are only shown for the currently 'followed' balloon ('follow_vehicle' variable)

[darksidelemm]

Some changes have been made and are in https://testing.v2.sondehub.org/

• Horizon rings are now only shown for the currently selected radiosonde.
• The "Show Horizon Rings" option is now removed, and horizon rings (for the single radiosonde) will now always be shown.