sandwich workflow

[stdavis]

We need to allow users to sandwich their data in between the base and overlay layers.

One way to do it would be to accept a new construction parameter (sandwich or cheese) that would accept a reference to the layer that the user wants to sandwich. The user would then be responsible to control visibility of the layer independent of the layer-selector widget.

[steveoh]

we would have to return a reference to the layer or accept an already created layer object.

[stdavis]

I think that accepting an already created layer object is the best bet. Seems the most simple workflow.