Add particle tracks

Adds a layer to the map to show particle tracks.

Currently working with static JSON data.  To generate the static data, run `tabs/tracker.py` as a script (requires [TracPy](https://github.com/kthyng/tracpy) to be installed).  In theory this could compute data as needed, but it is currently far too slow to do remotely (takes several minutes).

Particles turn gray as they leave the domain and are no longer tracked.

Known issue: date/time in legend does not update.  That data should be kept somewhere so it knows the time steps.  That should be refactored in common with the other layers, IMO.