docs: def nodes_minimum_distance_validation(self):
      """
      if minimum distance is specified, ensure node is not too close to other nodes;
      """
      if self.layer and self.layer.nodes_minimum_distance:
          minimum_distance = self.layer.nodes_minimum_distance
          # TODO - lower priority: do this check only when coordinates are changing
          near_nodes = Node.objects.exclude(pk=self.id).filter(geometry__distance_lte=(self.geometry, D(m=minimum_distance))).count()
          if near_nodes > 0:
              raise ValidationError(_('Distance between nodes cannot be less than %s meters') % minimum_distance)