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/Map_simple.cpp b/doc/snippets/Map_simple.cpp
new file mode 100644
index 0000000..423bb52
--- /dev/null
+++ b/doc/snippets/Map_simple.cpp
@@ -0,0 +1,3 @@
+int array[9];
+for(int i = 0; i < 9; ++i) array[i] = i;
+cout << Map<Matrix3i>(array) << endl;