Save the map zoom and center so it can be restored when switching platforms

[noaaroland]

Now when you select a new platform to plot, the map redraws to show the history of the locations of the newly selected platform. This causes the map center and zoom to be reset.

Probably the best way to fix this is to draw the platforms and the trace of the selected platform in two separate callbacks. In the second callback update the location trace without redrawing the whole map. This can be done by getting the map figure as state input to the second callback.

This can be unpacked when we tear out saving the state in the URL and start using the built-in deep linking capability.