Validate that faces in a shell don't intersect

[hannobraun]

About

The faces of a shell must not intersect, if the resulting shape is to be valid. This is currently neither documented nor enforced.

There should be a validation check, implemented for Shell, to make sure of this.

Implementation

The new validation check should go into validate::shell. The existing checks there (and the numerous checks in the sibling modules) can be used as a template for how to structure the new check.

Related Issues

There are a few related issues that are currently open:

• Separate geometry from topology #2116 and Consider uniform intermediate representation for geometry #2118 concern a fundamental re-design of how geometry is handled. If those were implemented first, that would simplify the implementation of this issue.
• Improve validation infrastructure #2157 concerns a refactor to 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.

Those issues aren't blockers, but if any work on them was ongoing by the time this issue was picked up, that would require coordination between the issues.

[hannobraun]

The validation code will most likely be replaced when the experimental architecture is merged back into the mainline code. This issue may still apply then (the experimental code has no validation code to speak of, so far), but I think it would be a waste of time to fix this before then.

Labeling as "blocked" until the experimental code is merged.