def read_chunk(filename, chunk_size, start):
    """
    Read a chunk of the trajectory file.

    Parameters
    ----------
    filename: string
        filename of the file to load
    chunk_size: int
        size of the chunk to load
    start: int
        frame to start the loading from

    Returns
    -------
    A tuple with the data for the read chunk (depending on what
    filetype.read returns)
    """
    with open(filename) as f:
        pos = f.tell()
        seek_pos = start
        rel_pos = seek_pos - pos
        f.seek(rel_pos, 1)
        result = f.read(chunk_size)
    if isinstance(result, tuple):
        return result
    else:
        return result,
