function LDU(L, D, U, a) {
    L[2] = a[2] / a[0];
    U[0] = a[0];
    U[1] = a[1];
    U[3] = a[3] - L[2] * U[1];
    return [L, D, U];
  }
Returns L, D and U matrices (Lower triangular, Diagonal and Upper triangular) by factorizing the input matrix
@param {ReadonlyMat2} L the lower triangular matrix
@param {ReadonlyMat2} D the diagonal matrix
@param {ReadonlyMat2} U the upper triangular matrix
@param {ReadonlyMat2} a the input matrix to factorize