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_noalias.cpp b/doc/snippets/MatrixBase_noalias.cpp
new file mode 100644
index 0000000..3b54a79
--- /dev/null
+++ b/doc/snippets/MatrixBase_noalias.cpp
@@ -0,0 +1,3 @@
+Matrix2d a, b, c; a << 1,2,3,4; b << 5,6,7,8;
+c.noalias() = a * b; // this computes the product directly to c
+cout << c << endl;