Parametrised intervals

Interval arithmetic rewritten to use parametrised intervals.
Tests pass.
I am actually not sure about ⊂, because it is ambiguous -- it is not clear if it allows equality or not.

I don't understand why `0.1 ⊊ a ` fails -- shouldn't this work by promotion? In any case, there is no reason for these operators to be defined except for sets (i.e., intervals).