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
31 jobs for mohr/fixBadCast in 20 minutes and 31 seconds (queued for 50 seconds)