[Development] Decrease stepsizes that would result in steps that are way too big (shape optimization)

[sblauth]

The Armijo line search procedure can take a long amount of time to find a suitable stepsize. This can be made (somewhat) faster by automatically decreasing the stepsize when the resulting update would be "too large" - for optimal control, this is hard to check, but for shape optimization a criterion based on the size of the geometry could be easily derived.