Pass Eigen arguments by const reference (#38)

Fixes #35.

cartographer/transform/transform.h

@@ -51,31 +51,33 @@ T GetYaw(const Rigid3<T>& transform) {
 // rotation as the given 'quaternion'.
 template <typename T>
 Eigen::Matrix<T, 3, 1> RotationQuaternionToAngleAxisVector(
-    Eigen::Quaternion<T> quaternion) {
-  quaternion.normalize();
+    const Eigen::Quaternion<T>& quaternion) {
+  Eigen::Quaternion<T> normalized_quaternion = quaternion.normalized();
   // We choose the quaternion with positive 'w', i.e., the one with a smaller
   // angle that represents this orientation.
-  if (quaternion.w() < 0.) {
+  if (normalized_quaternion.w() < 0.) {
     // Multiply by -1. http://eigen.tuxfamily.org/bz/show_bug.cgi?id=560
-    quaternion.w() *= T(-1.);
-    quaternion.x() *= T(-1.);
-    quaternion.y() *= T(-1.);
-    quaternion.z() *= T(-1.);
+    normalized_quaternion.w() *= T(-1.);
+    normalized_quaternion.x() *= T(-1.);
+    normalized_quaternion.y() *= T(-1.);
+    normalized_quaternion.z() *= T(-1.);
   }
-  // We convert the quaternion into a vector along the rotation axis with
-  // length of the rotation angle.
-  const T angle = T(2.) * atan2(quaternion.vec().norm(), quaternion.w());
+  // We convert the normalized_quaternion into a vector along the rotation axis
+  // with length of the rotation angle.
+  const T angle = T(2.) * atan2(normalized_quaternion.vec().norm(),
+                                normalized_quaternion.w());
   constexpr double kCutoffAngle = 1e-7;  // We linearize below this angle.
   const T scale = angle < kCutoffAngle ? T(2.) : angle / sin(angle / T(2.));
-  return Eigen::Matrix<T, 3, 1>(scale * quaternion.x(), scale * quaternion.y(),
-                                scale * quaternion.z());
+  return Eigen::Matrix<T, 3, 1>(scale * normalized_quaternion.x(),
+                                scale * normalized_quaternion.y(),
+                                scale * normalized_quaternion.z());
 }
 
 // Returns a quaternion representing the same rotation as the given 'angle_axis'
 // vector.
 template <typename T>
 Eigen::Quaternion<T> AngleAxisVectorToRotationQuaternion(
-    Eigen::Matrix<T, 3, 1> angle_axis) {
+    const Eigen::Matrix<T, 3, 1>& angle_axis) {
   T scale = T(0.5);
   T w = T(1.);
   constexpr double kCutoffAngle = 1e-8;  // We linearize below this angle.