make the storm dot user-movable

[aappling-usgs]

Let users click and drag the storm dot along the storm track to move forward and back in space-time

[jordansread]

I have always thought this was a good idea, and one way to do it is to make the dot draggable and make it confined to the range and the locations of the storm track.

But, this makes it a little tricky for storms that lose the track at the beginning or end of the animation (e.g., maria )

An alternative is to expose a small slider bar for a few seconds when someone clicks play or pause, which would allow you to drag the time slider (similar to a lot of browser gif players).

It would be a bit easier to code since the curser values are limited to x space and locked in y. I think this is much less ideal for the storms that fit the nice pattern (e.g., irma ) where you want to grab the dot, but also a bit more re-usable for the other storms and also for non-storm vizzies that could use this functionality to advance/retreat time in an animation.