def add_coordinate(coordinate)
      raise ArgumentError, "Invalid argument 'coordinate'. Expected Coordinate, got #{coordinate.class}." unless coordinate.is_a?(Coordinate)
      @coordinates << coordinate unless @coordinates.include?(coordinate)
    end
def add_coordinate(coordinate)
      raise ArgumentError, "Invalid argument 'coordinate'. Expected Coordinate, got #{coordinate.class}." unless coordinate.is_a?(Coordinate)
      @coordinates << coordinate unless @coordinates.include?(coordinate)
    end
Adds a Coordinate instance to this Contour.

 @param [Coordinate] coordinate a coordinate instance to be associated with this contour