Tests for 0.8

The important commit here is the last one. I will rebase on master once the many recent changes to `Model` are merged.

This adds some tests (e.g., of `result.eval`) and applies them to several built-in models as well as a user-defined model. Previously, testing of the built-in models was very light.

Some models are still not tested at all. We should have at least one or two simple tests covering all the built-in models, so this is a WIP.