swap visualization techniques

[djairoh]

implement a hook to switch between layers on the go. Likely will require sharing vtkPolydata and/or vtkPoints between layers.

[djairoh]

similarly but not identically; add functionality to switch between different types of channel modulation (within one layer). Actually this might be a bigger challenge than the swapping of layers;

• add a function to Layer to setModulationMode(int mode)?
• Have different layers and just pretend it's only changing modulation?

Either way this is going to involve another callback.