fix doc generation with doxygen 1.14 & 1.15 libeigen/eigen!2095 Closes #2976
Eigen is a C++ template library for linear algebra: matrices, vectors, numerical solvers, and related algorithms.
For more information go to http://eigen.tuxfamily.org/ or https://libeigen.gitlab.io/docs/.
For pull request, bug reports, and feature requests, go to https://gitlab.com/libeigen/eigen.