Review tolerance for determining whether or not bounds are fixed.

[nbelakovski]

At the moment we use the following code to determine whether or not a lower bound is equal to a given upper bound:

tol = get_arrays_tol(lb, ub)
_fixed_idx = (
    (lb <= ub)
    & (np.abs(lb - ub) < tol)
)

In this case get_arrays_tol provides a tolerance that is scaled based on the length of the provided arrays and the largest number in the array, but this can lead to a pathological case like the following two arrays

lb: (1, 2)
ub: (very large number, 3)

If the large number is large enough, the tolerance will be large and we would consider the number bound by 2 and 3 to be fixed.

One alternative approach is to just always use machine epsilon. Another is to use an array of tolerances. This latter approach would work better for a case like the following:

lb: (very large number)
ub: (very large number) + 1

Where the difference is larger than eps but where we might want to consider the bounds to effectively be fixed.