Discrete Color Bar Improvements

[nicolemarsaglia]

MIR gave us a pseudo-discrete color bar that forces the ticks to be whole numbers, but overall still uses a continuous color table.

This can be improved but will most likely be finnicky, hair-pulling work playing around with viskores (vtk-m) color table options.

As of now it can handle material ids that do not start at one, but does not eliminate nonexistent material ids (second image: the 2-8 should only be the even numbers). Nor will it grow the color bar if there are a lot of materials (third image).

[nicolemarsaglia]

Gets set in annotator: https://github.com/Alpine-DAV/ascent/blob/task/2024_05_vtkm_material_interface_recon/src/libs/vtkh/rendering/Annotator.cpp#L47