[eigen] Welcome David Tellenbach as an Eigen maintainer

[Rasmus Munk Larsen]

On behalf of the maintainers of Eigen, I would like to welcome David
Tellenbach to the team. David has been an active contributor for a while,
and recently has worked on setting up continuous integration on Gitlab, as
well as helping review PRs. We are delighted to welcome him to the team and
look forward to working with him and the community to get Eigen development
back up to speed after a period of relative calm.