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/MatrixBase_col.cpp b/doc/snippets/MatrixBase_col.cpp
new file mode 100644
index 0000000..87c91b1
--- /dev/null
+++ b/doc/snippets/MatrixBase_col.cpp
@@ -0,0 +1,3 @@
+Matrix3d m = Matrix3d::Identity();
+m.col(1) = Vector3d(4,5,6);
+cout << m << endl;