message: edge collapse doesn't use eigen anymore
diff: #include <gudhi/distance_functions.h>
  #include <gudhi/Simplex_tree.h>
  #include <gudhi/Points_off_io.h>
  -#ifdef GUDHI_USE_EIGEN3
  #include <gudhi/Flag_complex_edge_collapser.h>
  -#endif
  #include <iostream>
  #include <vector>
  @@ -164,7 +162,6 @@ class Simplex_tree_interface : public Simplex_tree<SimplexTreeOptions> {
  }
  Simplex_tree_interface* collapse_edges(int nb_collapse_iteration) {
  -#ifdef GUDHI_USE_EIGEN3
  using Filtered_edge = std::tuple<Vertex_handle, Vertex_handle, Filtration_value>;
  std::vector<Filtered_edge> edges;
  for (Simplex_handle sh : Base::skeleton_simplex_range(1)) {
  @@ -178,7 +175,7 @@ class Simplex_tree_interface : public Simplex_tree<SimplexTreeOptions> {
  }
  for (int iteration = 0; iteration < nb_collapse_iteration; iteration++) {
  - edges = Gudhi::collapse::flag_complex_collapse_edges(edges);
  + edges = Gudhi::collapse::flag_complex_collapse_edges(std::move(edges));
  }
  Simplex_tree_interface* collapsed_stree_ptr = new Simplex_tree_interface();
  // Copy the original 0-skeleton
  @@ -190,9 +187,6 @@ class Simplex_tree_interface : public Simplex_tree<SimplexTreeOptions> {
  collapsed_stree_ptr->insert({std::get<0>(remaining_edge), std::get<1>(remaining_edge)}, std::get<2>(remaining_edge));
  }
  return collapsed_stree_ptr;
  -#else
  - throw std::runtime_error("Unable to collapse edges as it requires Eigen3 >= 3.1.0.");
  -#endif
  }
  // Iterator over the simplex tree
  