PDESolver reuse matrix doesn't work in parallel

Can be tested with !3380 (merged) by adding

assembler.assembleJacobianAndResidual(sol);
solver.reuseMatrix(true);

I suspect that the parallel solver adapts the matrix every time and this operation is probably additive. I this case, we would need to let the solver know that the matrix is not to be changed because it's already been prepared for a parallel solve.