[examples] Use pattern jacobian = referenceJacobian * transpose(jit);

Use new utility transpose() from corresponding branch in dune-common. In order to make this even more idiomatic

• Rename 'gradient' to 'jacobian'
• Use matrices for Jacobians instead of vectors for gradients
• Introduce helper dot(a,b) for matrix dot product
• Introduce helper trace(A) for div = trace(jacobian)