code: def point3d(value, lon, lat, depth):
      
      return longitude(lon), latitude(lat), positivefloat(depth)
docs: This is used to convert nodes of the form
      <hypocenter lon="LON" lat="LAT" depth="DEPTH"/>
  
      :param value: None
      :param lon: longitude string
      :param lat: latitude string
      :returns: a validated triple (lon, lat, depth)