Should path indices start from 1?

[jni]

Working with @kevinyamauchi on displaying skeletons in napari, we realized that the labels layer is a good fit for many use cases: we can create a labels layer with the path id as the label, and have a properties table with the Skeleton summary produced by summarize. That allows one to visualize paths by many different properties.

The problem is that right now the path ids start from 0, and 0-labels are transparent. We can circumvent this by adding a +1 to the labels and adding an index column to the table, but that's clunky.