When the function used for metric does not satisfy the triangle inequality, monodromy can be faulty. This PR adds an option triangle_inequality, such that triangle_inequality = false overrides all checks based on the triangle inequality.

I guess the function is then also not a metric 😅

Yes, true. But there can be situations where it makes sense to pass a function that measures distances, but is not a metric in the mathematical sense. Should we rename metric to distance in UniquePoints?