BUG: wrong bounding of sensitivity contituents in case of many control boxes (Fixes #1549)

When more than one volumetric B-Splines control boxes are present, the sensitivity constituents corresponding to the non-active design variables were not bounded(zeroed) correctly. The resultant sensitivities, used in the optimization, were bounded correctly, so this was more a bug pertaining to the output file of the sensitivities rather than a functional one.