Add a layer selection tool

[johntyree]

This adds the ability to toggle the velocity and salinity layers.

It still suffers from a bad z-ordering. The initial frame renders correctly, but subsequent frames render the contours over the vectors. I have tried several magic tricks without success and encourage anyone interested to volunteer ideas.

The layerSelectorControl takes care of adding and remove layers from the map and infers its initial state by which layers are present therein. I augment it here with a method to hook up listeners that also set mapView.visibleLayers appropriately, which should prevent layers from trying to download data or redraw themselves when they aren't visible.

Problems exposed by this PR but not obviously caused by it:

• z-order of layers is wrong after first call to mapView.redraw()
• animating both layers at once is very slow and causes redraw events to get badly backed up. (#33)
• It's difficult to tell, but it also looks like it suffers from #31 and/or #39 when both are visible. It's hard to tell because of #33.

Assuming these problems are tolerable for now, this fixes #47.

[johntyree]

If it's too broken to have the option of displaying both layers, we could temporarily fake radio buttons out of the check boxes or simply not attach the control to the map while leaving the logic in place.

[kjordahl]

I like the layer icon. For now I think it's OK to expose both. That's most useful for testing.

[kjordahl]

:+1: