Add ability to toggle raster layers

[juancastilla]

as suggested by @kbefus

[kbefus]

The water table depth png is toggle-able in the layer list. I can make a png of the water table head from Inge as well. I'm not sure how much the png loading adds the app load time. I think png (~3MB) is the way to go vs tif (~30 MB). We might also be able to add a colorbar in photo-editing software (but need to keep resolution and number of pixels the same)