Restore example models

[hannobraun]

With the recent changes, we lost the example models that were formerly in the repository, since those were based on the now removed app and other deprecated components.

Restoring them involves expanding and cleaning up the kernel API (the former models used a high-level API that is no longer part of the project). So far, I've restored the cuboid model, and I'm working on restoring the rest of them.

I've been working on this for a while, but have been a bit negligent in opening issues for what I'm doing. Sorry! Currently, I'm working on computing the bounding volume from boundary representation. Formerly, the bounding volume was computed from the output of the removed high-level API.