BactrianUpDownOperator modifies the wrong index when elementWise is true

[bjoelle]

l95 and l105 have this code p.setValue(p.getValue(index) * scale); which as far as I can tell will modify the value at position 0 regardless of what index is set to
the equivalent code in UpDown operator is p.setValue(index, p.getValue(index) * scale); as expected

[rbouckaert]

Good point! Should be fixed by c9862c5