message: Reuse vector
  Reuse + copy should be slightly faster than regrowing each time (and moving)
diff: @@ -73,15 +73,17 @@ persistent_cohomology::Persistent_cohomology<FilteredComplex, persistent_cohomol
  std::vector<std::pair<std::vector<int>, std::vector<int>>> persistence_pairs;
  auto const& pairs = Base::get_persistent_pairs();
  persistence_pairs.reserve(pairs.size());
  - for (auto pair : pairs) {
  std::vector<int> birth;
  + std::vector<int> death;
  + for (auto pair : pairs) {
  + birth.clear();
  if (get<0>(pair) != stptr_->null_simplex()) {
  for (auto vertex : stptr_->simplex_vertex_range(get<0>(pair))) {
  birth.push_back(vertex);
  }
  }
  - std::vector<int> death;
  + death.clear();
  if (get<1>(pair) != stptr_->null_simplex()) {
  death.reserve(birth.size()+1);
  for (auto vertex : stptr_->simplex_vertex_range(get<1>(pair))) {
  @@ -89,7 +91,7 @@ persistent_cohomology::Persistent_cohomology<FilteredComplex, persistent_cohomol
  }
  }
  - persistence_pairs.emplace_back(std::move(birth), std::move(death));
  + persistence_pairs.emplace_back(birth, death);
  }
  return persistence_pairs;
  }
  