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/AngleAxis_mimic_euler.cpp b/doc/snippets/AngleAxis_mimic_euler.cpp
new file mode 100644
index 0000000..456de7f
--- /dev/null
+++ b/doc/snippets/AngleAxis_mimic_euler.cpp
@@ -0,0 +1,5 @@
+Matrix3f m;
+m = AngleAxisf(0.25*M_PI, Vector3f::UnitX())
+  * AngleAxisf(0.5*M_PI,  Vector3f::UnitY())
+  * AngleAxisf(0.33*M_PI, Vector3f::UnitZ());
+cout << m << endl << "is unitary: " << m.isUnitary() << endl;