CGAL <-> Eigen conversions

[hermannsblum]

as was previously part of the architect model in cadify

[jstiefel]

@hermannsblum Since we don't have any dependencies on Eigen in cgal_conversions, does it make more sense to add these in cpt_utils or add Eigen dependencies to cgal_conversions?

[hermannsblum]

let's add the eigen dependency to cgal_conversions for now, we can see later if we need to declutter.

[jstiefel]

I started a new branch (feature/cgal_eigen_conversions) for adding CGAL<->Eigen conversions in eigen_conversions.cc of cgal_conversions. Feel free to add functions:

https://github.com/ethz-asl/cad-percept/tree/feature/cgal_eigen_conversions

[hermannsblum]

@jstiefel You can do a PR for the 2 conversion functions right away, that is actually nicer than waiting for a big chunk