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.