Commit formally makes VectorLaplaceOperator smoothable
The VectorLaplaceOperator class now publically inherits from - GSSmoothable - GSBackwardsSmoothable - SORSmoothable - SORBackwardsSmoothable and implements the associated pure virtual methods. The implementation in turn checks that the diagonal sub-operators have the corresponding interface and delegates work to them. Note: Of course a P2ElementwiseVectorLaplaceOperator is still not truely SORSmoothable. However, that will only be recognised on the sub-operator level.