title: Eigen::Quaterniond to Eigen::AngleAxis
body: <p>I want to convert an Eigen::Quaterniond to a Eigen::Vector3d representing a rotation vector (angle * axis). My thought was to convert it into an Eigen::AngleAxis format and then convert that to Eigen::Vector3d . </p>
  
  <p>Is there a direct function to go from Eigen::Quaterniond to Eigen::AngleAxis?</p>