Immutable palettes

[mike-mcgann]

Description

Fixes nasa-gibs/worldview#1109.

Added "immutable" parameter to:

• Landsat_WELD_CONUS_5_Year_Tree_Cover

Any other layers that should be added?

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@nasa-gibs/worldview

[minniewong]

1. When I load WELD Tree cover, the legend is present, if I click around a lot on that layer in the layer list, the colorbar legend disappears.
2. We might want to move WELD Tree cover to "overlays" instead of keeping it at "baselayers" - thoughts?

[mike-mcgann]

Actually, I think it needs to be an overlay. As a baselayer, the map will ignore everything below it since it assumes baselayers cover the full extent and are opaque. Changed to overlay.

For item 1, I was able to generate that bug simply by trying to move the layer. Making it an overlay solved the problem. Something in the code might assume colorbars are not allowed for baselayers.