big improvements to tutorial, especially page 2 (matrix arithmetic).

add placeholders for some 'special topic' pages.

doc/examples/tut_arithmetic_add_sub.cpp

@@ -0,0 +1,22 @@
+#include <iostream>
+#include <Eigen/Dense>
+
+using namespace Eigen;
+
+int main()
+{
+  Matrix2d a;
+  a << 1, 2,
+       3, 4;
+  MatrixXd b(2,2);
+  b << 2, 3,
+       1, 4;
+  std::cout << "a + b =\n" << a + b << std::endl;
+  std::cout << "a - b =\n" << a - b << std::endl;
+  std::cout << "Doing a += b;" << std::endl;
+  a += b;
+  std::cout << "Now a =\n" << a << std::endl;
+  Vector3d v(1,2,3);
+  Vector3d w(1,0,0);
+  std::cout << "-v + w - v =\n" << -v + w - v << std::endl;
+}