def appendData(filename, data):
    """Append a binary set of data to the end of a MRC file. This should only be used in conjunction with
    writeHeader() above. 
    
    Parameters
    ----------
        filename: str
            Name of the MRC file with pre-initiated header and some data already written.
        data: ndarray
            Data to append to the file.
    
    """
    with open(filename, 'ab') as fid:
        fid.seek(0, 2)
        fid.write(data)
