Initial commit

Eigen3/doc/snippets/HessenbergDecomposition_packedMatrix.cpp

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+Matrix4d A = Matrix4d::Random(4,4);
+cout << "Here is a random 4x4 matrix:" << endl << A << endl;
+HessenbergDecomposition<Matrix4d> hessOfA(A);
+Matrix4d pm = hessOfA.packedMatrix();
+cout << "The packed matrix M is:" << endl << pm << endl;
+cout << "The upper Hessenberg part corresponds to the matrix H, which is:" 
+     << endl << hessOfA.matrixH() << endl;
+Vector3d hc = hessOfA.householderCoefficients();
+cout << "The vector of Householder coefficients is:" << endl << hc << endl;