Compute the inverse of an interval
The case where xl is very slightly positive but should be very
slightly negative (or xu is very slightly negative but should be
very slightly positive) should not be an issue. Suppose xu is 2 and
xl is 1e-15 but should be -1e-15. The bounds obtained from this
function will be [0.5, 1e15] or [0.5, inf), depending on the value
of feasibility_tol. The true bounds are (-inf, -1e15] U [0.5, inf),
where U is union. The exclusion of (-inf, -1e15] should be
acceptable. Additionally, it very important to return a non-negative
interval when xl is non-negative.