First start of frame (SOFn) marker in this sequence.

def sof(self):
        """
        First start of frame (SOFn) marker in this sequence.
        """
        for m in self._markers:
            if m.marker_code in JPEG_MARKER_CODE.SOF_MARKER_CODES:
                return m
        raise KeyError('no start of frame (SOFn) marker in image')