Unhide circularities feature

[alexdewar]

At present, the only way to test a model with circularities is to enable the please_give_me_broken_results option in model.toml.

Unhiding this option won't affect any existing model without circularities. The only possible problem is that it may not work for every model with circularities, however, if it doesn't work the user will get an error message rather than bad results.

(Is this right @tsmbland?)

Based on this, I think we should just enable this feature unconditionally, for a few reasons:

1. Existing models will work as before
2. Models with circularities may not run to completion, but won't give bad results either, so this is still an improvement
3. The "broken results" option is really there to hide other, potentially dangerous options, rather than just gate all functionality that we're not sure about. There's currently no way to test one of the other hidden options, like NPV, without also enabling the circularities feature, which seems wrong. I know this won't actually matter in this case, but we don't want to be turning on/off all experimental features unconditionally going forward.

We could also consider having a way to enable experimental features, as @dalonsoa suggested, but I don't think that's necessary here.*

Do you agree @tsmbland?

*If we did want to do that, we could have something like:

# model.toml
please_give_me_broken_results = true
experimental_features = ["circularities", "flux_capacitor"] # etc.

Might be worth doing for the next experimental feature we add!

[tsmbland]

Except that the example model doesn't work 🫢. Pretty sure it used to work so I'll have to investigate...