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/snippets/Tutorial_commainit_01.cpp b/doc/snippets/Tutorial_commainit_01.cpp
new file mode 100644
index 0000000..47ba31d
--- /dev/null
+++ b/doc/snippets/Tutorial_commainit_01.cpp
@@ -0,0 +1,5 @@
+Matrix3f m;
+m << 1, 2, 3,
+     4, 5, 6,
+     7, 8, 9;
+std::cout << m;