Feature request: allow to visualize iso-surfaces in structures

[Luthaf]

Allowing to visualize iso-surfaces, e.g. for electronic density, would be very nice. Most visualization engines support something like this, so the main issues on our side are

• Define how to store the 3D data in the input JSON file/in-memory dataset
• Add options to configure the iso-surface rendering (colors, iso-value, …)

[ceriottm]

Just noting that with #292 and #290 this is possible by hard-coding the isosurfaces as custom shapes. Probably NOT how we want to do this so the issue stays open, but if someone is desperate, now it should be possible to display orbital and densities as isocontours.