Squashed 'third_party/eigen/' content from commit 61d72f6

Change-Id: Iccc90fa0b55ab44037f018046d2fcffd90d9d025
git-subtree-dir: third_party/eigen
git-subtree-split: 61d72f6383cfa842868c53e30e087b0258177257
diff --git a/doc/examples/Tutorial_ArrayClass_addition.cpp b/doc/examples/Tutorial_ArrayClass_addition.cpp
new file mode 100644
index 0000000..480ffb0
--- /dev/null
+++ b/doc/examples/Tutorial_ArrayClass_addition.cpp
@@ -0,0 +1,23 @@
+#include <Eigen/Dense>
+#include <iostream>
+
+using namespace Eigen;
+using namespace std;
+
+int main()
+{
+  ArrayXXf a(3,3);
+  ArrayXXf b(3,3);
+  a << 1,2,3,
+       4,5,6,
+       7,8,9;
+  b << 1,2,3,
+       1,2,3,
+       1,2,3;
+       
+  // Adding two arrays
+  cout << "a + b = " << endl << a + b << endl << endl;
+
+  // Subtracting a scalar from an array
+  cout << "a - 2 = " << endl << a - 2 << endl;
+}