message: add control to not duplicate boundary relation
diff: @@ -214,11 +214,21 @@ namespace RINGMesh {
  const gmme_id& in_boundary,
  const gmme_id& boundary )
  {
  + index_t possibly_existing_relationship_id = find_boundary_relationship(
  + in_boundary, boundary );
  + if( possibly_existing_relationship_id != NO_ID ) {
  + Logger::warn( "Relationship" )
  + << "The relation between the in boundary " << in_boundary
  + << " and the boundary " << boundary
  + << " already exists. Nothing is done";
  + return possibly_existing_relationship_id;
  + } else {
  index_t relationship_id =
  static_cast< index_t >( boundary_relationships_.size() );
  boundary_relationships_.emplace_back( in_boundary, boundary );
  return relationship_id;
  }
  + }
  index_t find_boundary_relationship(
  const gmme_id& in_boundary,
  @@ -267,11 +277,20 @@ namespace RINGMesh {
  const gmge_id& parent,
  const gmme_id& child )
  {
  + index_t possibly_existing_relationship_id =
  + find_parent_child_relationship( parent, child );
  + if( possibly_existing_relationship_id != NO_ID ) {
  + Logger::warn( "Relationship" ) << "The relation between the parent "
  + << parent << " and the child " << child
  + << " already exists. Nothing is done";
  + return possibly_existing_relationship_id;
  + } else {
  index_t relationship_id =
  static_cast< index_t >( parent_child_relationships_.size() );
  parent_child_relationships_.emplace_back( parent, child );
  return relationship_id;
  }
  + }
  index_t find_parent_child_relationship(
  const gmge_id& parent,
  