New plot: signal ODF

This has resulted in this type of plot (which I don't think is particularly helpful and the code is very ugly and difficult to extend. Will consider moving to Panel+Plotly if we can.

I don't know about you but I think this DIVE app is badly missing some 3D plotting. I'll try to fix this and add aa signal ODF - hopefully something we can spin around.

• 3D glyph
• Slider that changes the bvalue.
• Maybe also tick box that turns it into Radon transformed signal? Not sure.

Note: this will be useful when the FOD stuff has been implemented :)