Parametrised intervals

Interval arithmetic rewritten to use parametrised intervals.
Tests pass.
I agree; it just matters for the promotion rules. 

Yet, I think the question should be reformulated as "is it a `Number` or a `Set`?" My concern is not on the promotion rules, but for defining things like `a + ∅` (or any other arithmetic operation with an empty interval), which make sense if intervals are `Number`-like objects, or things like `∅ ∩ a`, which make sense among sets.