Allow for easily patching examples other than `simple` in test code

[alexdewar]

The way the API is written currently, you can only easily work with patched models based on simple. In the longer run, we will inevitably need to patch other examples for tests, so it makes sense to have some additional, more generic helpers (on which the existing ones for the simple model can be based).

[alexdewar]

While we're at it we could add an info field: #1081 (comment)

[alexdewar]

It would be good to have a test for the circularities model with NPV