message: Use std::optional to check C++17 support
diff: #include <Eigen/Eigen>
  #include <Eigen/src/Core/util/Macros.h> // for EIGEN_VERSION_AT_LEAST
  -#include <boost/optional.hpp>
  #include <boost/iterator/transform_iterator.hpp>
  #include <boost/range/adaptor/transformed.hpp>
  #include <boost/range/counting_range.hpp>
  #include <cmath> // for std::sqrt
  #include <string>
  #include <cstddef> // for std::size_t
  +#include <optional>
  #ifdef GUDHI_USE_TBB
  #include <tbb/parallel_for.h>
  @@ -994,7 +994,7 @@ class Tangential_complex {
  // circumspheres of the star of "center_vertex"
  // If th the m_max_squared_edge_length is set the maximal radius of the "star sphere"
  // is at most square root of m_max_squared_edge_length
  - boost::optional<FT> squared_star_sphere_radius_plus_margin = m_max_squared_edge_length;
  + std::optional<FT> squared_star_sphere_radius_plus_margin = m_max_squared_edge_length;
  // Insert points until we find a point which is outside "star sphere"
  for (auto nn_it = ins_range.begin(); nn_it != ins_range.end(); ++nn_it) {
  @@ -1036,7 +1036,7 @@ class Tangential_complex {
  // Let's recompute squared_star_sphere_radius_plus_margin
  if (triangulation.current_dimension() >= tangent_space_dim) {
  - squared_star_sphere_radius_plus_margin = boost::none;
  + squared_star_sphere_radius_plus_margin = std::nullopt;
  // Get the incident cells and look for the biggest circumsphere
  std::vector<Tr_full_cell_handle> incident_cells;
  triangulation.incident_full_cells(center_vertex, std::back_inserter(incident_cells));
  @@ -1044,7 +1044,7 @@ class Tangential_complex {
  cit != incident_cells.end(); ++cit) {
  Tr_full_cell_handle cell = *cit;
  if (triangulation.is_infinite(cell)) {
  - squared_star_sphere_radius_plus_margin = boost::none;
  + squared_star_sphere_radius_plus_margin = std::nullopt;
  break;
  } else {
  // Note that this uses the perturbed point since it uses
  @@ -2030,7 +2030,7 @@ class Tangential_complex {
  // and their center vertex
  Stars_container m_stars;
  std::vector<FT> m_squared_star_spheres_radii_incl_margin;
  - boost::optional<FT> m_max_squared_edge_length;
  + std::optional<FT> m_max_squared_edge_length;
  #ifdef GUDHI_TC_USE_ANOTHER_POINT_SET_FOR_TANGENT_SPACE_ESTIM
  Points m_points_for_tse;
  