Opacity binning in voxel export

[Carifio24]

While we don't make multiple materials with the exact same opacity for the voxel export, there will generally be a lot of close-but-different opacity values for these. This means that we're adding a lot of materials to the exported file, many of which probably aren't distinguishable. We should consider adding a voxel export option that allows a user to specify how many opacity bins/what resolution they want to have for opacity. For example, if a user set this value to 100, we would round to the nearest 0.01 when determining the final voxel opacity.