BUG: globalSum needed in the directional derivative of the merit function

Summary

When the derivative of the merit function is computed the global (instead of the local) sum of the objective derivative and the correction should be calculated.

This bug does not actually affect the current functionality of the code since, with volumetric B-Splines, all design variables (and their derivatives and corrections) are known by all processors. It will, however, affect cases in which the design variables are distributed, like topology optimisation or when considering the movement of each boundary point as a design variable in shape optimisation.