Returns the next data value.
    :return: (float|int) the next data value

def next(self):
    """
    Returns the next data value.
    :return: (float|int) the next data value
    """
    out = self.peek()[self._headers.index(self._field)]
    self._cursor += 1
    if out is not None:
      self._lastValue = out
    return out