Rewrite tutorial section on solving linear systems

doc/snippets/Tutorial_solve_singular.cpp

@@ -0,0 +1,9 @@
+Matrix3f A;
+Vector3f b;
+A << 1,2,3,  4,5,6,  7,8,9;
+b << 3, 3, 4;
+cout << "Here is the matrix A:" << endl << A << endl;
+cout << "Here is the vector b:" << endl << b << endl;
+Vector3f x;
+A.partialLu().solve(b, &x);
+cout << "The solution is:" << endl << x << endl;