Bug 824: improve accuracy of Quaternion::angularDistance using atan2 instead of acos.
Bug 824: improve accuracy of Quaternion::angularDistance using atan2 instead of acos.

madlib/eigen_backup,rotorliu/eigen,madlib/eigen_backup,rotorliu/eigen,madlib/eigen_backup,rotorliu/eigen,madlib/eigen_backup,rotorliu/eigen