C++ - designing an accelerated 'gradient descent' algorithm.

Working on a redesign of class Scale (part 8)

Next problem: continuing in the "same direction" is necessary, because
we continue to see no maximum in the approximation (now rejecting the
maximum of the cubic because that cubic looks way too much like a parabola,
which only has a minimum - that one we just came from). But exactly how can
we tell which direction it is? (hint: it isn't `hrestriction_`).