[Contraction] Remove dependency with CGAL

[VincentRouvreau]

Contraction c++ module is only requiring CGAL in order to use CGAL::Modifiable_priority_queue. This dependency could be removed by using some boost heap as the riority queue has to be modifiable. The other thing to understand is the difference between different types of queues as boost::relaxed_heap, or CGAL::CGAL_BOOST_PAIRING_HEAP is not giving the same result as CGAL::CGAL_BOOST_PENDING_RELAXED_HEAP (and the former version of CGAL < 5.5).