libeigen/eigen!2509 Closes #2868 Co-authored-by: Rasmus Munk Larsen <rmlarsen@gmail.com>
* use SelfAdjointView instead of Eigen2's SelfAdjoint flag. * add tests and documentation. * allow eigenvalues() for non-selfadjoint matrices. * they no longer depend only on SelfAdjointEigenSolver, so move them to a separate file