How can we symmetrize the tensor associated with structure by averaging the elastic constant tensor 𝐶 transformed under all the symmetry operations S in the point group of the underlying lattice? Could you explain the process using the equation 𝐶𝑖𝑗𝑘𝑙=𝑠𝑖𝑖𝑠𝑗𝑗𝑠𝑘𝑘𝑠𝑙𝑙𝐶𝑖𝑗𝑘𝑙Cijkl=siisjjskksllC"ijkl?