hide internal stuff from doxygen by using #ifndef EIGEN_PARSED_BY_DOXYGEN.
hide internal stuff from doxygen by using #ifndef EIGEN_PARSED_BY_DOXYGEN.

Sadly, \internal didn't hide internal stuff, it only hides the documentation!

robustrobotics/eigen,robustrobotics/eigen,robustrobotics/eigen,robustrobotics/eigen