Improve 3d figure legend

[jgieseler]

The legend in all 3d figures created with plotly (output from plot_3d & pfss_3d) should be updated to correctly include the markers='number' # or 'letters' option (i.e., having numbers/letters on the markers) and provide an arrow for the "reference" entry. Unfortunately, as both this plotting options are hacked into plotly manually, this would (at the moment) also need some amount of manual work.