Add ability to visualize orbital projection of bands in electronic band structure plots

[janosh]

This suggestion comes from @munrojm.

We could take inspiration from https://github.com/SMTG-UCL/sumo (shout out to @utf 😄) and add an option to color bands by their orbital composition. See this example from the sumo gallery:

[mkhorton]

Yes, definitely on the wish-list!

Last time I looked into this, these were the constraints:

• plotly (and other tools that ultimately rely on SVG) cannot support gradient lines, like matplotlib does, so we're limited and cannot use that style
• The option of many individual circles of various sizes (as per the sumo example) might work, but we have not benchmarked -- will it be slow for large plots?
• If the plot is for a spin-polarized bandstructure, how do we want to distinguish between spin, while also doing the orbital projections?

It would be great to support this feature on MP.