Parametrised intervals

Interval arithmetic rewritten to use parametrised intervals.
Tests pass.
I think we should be a bit more careful with respect to type-stability (in the sense that `2^(-1)` throws a `DomainError`). I guess this will not be too frequent, since I don't think we are interested mainly in intervals of integer values...

In any case, line 5 should be
```julia
n < zero(n) && return inv( a^(-n) )
```