Validate that shells within a solid don't intersect

[hannobraun]

Current Situation

Shell defines a boundary within a 3-dimensional space. Solid defines a solid object, which is defined as a collection of shells. These shells might be completely distinct, forming multiple bodies of the solid, or they might be nested inside of each other, forming cavities within a body.

The shells themselves must not intersect, however. That would make the solid invalid. This is neither documented nor enforced right now. We need a validation check for that.

Implementation

The new validation check would go into validate::solid. There are already other validation check there, as well as numerous more in sibling modules, which can be used as a template for the new one.

Related Issues

There are a few related issues that are currently open:

• Separate geometry from topology #2116 and Consider universal intermediate representation for geometry #2118 are about a fundamental redesign to how geometry is handled. If those were implemented first, this would simplify the implementation of this issue.
• Improve validation infrastructure #2157 is about a refactor of the validation infrastructure. If that were implemented first, it would change where the new validation check would go, and how specifically it would be implemented.

None of those issues are blockers for this one, but if work on them was ongoing by the time this issue is picked up, that would require coordination between the issues.