def write(filename, update_time = true)
      @time = Time.now if @time.nil? || update_time
      @name ||= File.basename(filename)
      doc = generate_xml_doc
      File.open(filename, 'w+') { |f| f.write(doc.to_xml) }
    end
def write(filename, update_time = true)
      @time = Time.now if @time.nil? || update_time
      @name ||= File.basename(filename)
      doc = generate_xml_doc
      File.open(filename, 'w+') { |f| f.write(doc.to_xml) }
    end
Serialize the current GPXFile to a gpx file named <filename>.
 If the file does not exist, it is created.  If it does exist, it is overwritten.