Add eigen library to cfsdeps

Description

I need to add the Eigen library to the CFSdeps: https://eigen.tuxfamily.org/index.php?title=Main_Page

This library is used by the CGAL Jet-fitting class that I use in the curvilinear PML to determine the approximate properties of a surface. https://doc.cgal.org/latest/Jet_fitting_3/index.html

Check before assigning to a maintainer for review

• the history is clean
• code is well documented and understandable
• there is a testcase (in case of new functionality)
• every commit passes the pipeline

Maintainer checks before merge

• Review is approved, and all comments are resolved
• Check for Testsuite changes, if yes check
  • Testsuite merge request merges fast-forward
  • Testsuite-submodule SHA of every CFS commit references a commit in Testsuite Merge Request
  • Testsuite-submodule SHA of last CFS commit points to HEAD of corresponding Testsuite branch
• Pipeline passes for every commit
  • all stable tests pass
  • testsuite tests run
  • new tests are actually running (e.g. check if they appear on CDash)