Make function to match napari's labels colormap in shapes

[jni]

The 3D visualisation tutorial uses the spectral colormap together with random path labels to get a categorical visualisation. However, it would be convenient to make the categorical colormap match the napari labels colormap. One way to achieve this might be:

• create the path feature table (skan.summarize)
• add a column that uses napari's low_discrepancy_image to map the path_id + 1 to the same float that napari is using
• use the napari labels colormap directly on the shapes layer