Fixes bad_cast issue in P2P1ElementwiseBlendingStokesOperator
The sub-operators of P2ElementwiseBlendingVectorLaplaceOperator are of type P2ElementwiseBlendingLaplaceOperator and not P2ElementwiseLaplaceOperator.
Maybe it would be wiser to cast to P2ElementwiseBlendingVectorLaplaceOperator::scalarOpType.