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/TopicAliasing_mult1.cpp b/doc/snippets/TopicAliasing_mult1.cpp
new file mode 100644
index 0000000..cd7e900
--- /dev/null
+++ b/doc/snippets/TopicAliasing_mult1.cpp
@@ -0,0 +1,4 @@
+MatrixXf matA(2,2); 
+matA << 2, 0,  0, 2;
+matA = matA * matA;
+cout << matA;