'Returns the record at given index.
Example usage:
>>> record = mx.recordio.MXIndexedRecordIO(\'tmp.idx\', \'tmp.rec\', \'w\')
>>> for i in range(5):
...     record.write_idx(i, \'record_%d\'%i)
>>> record.close()
>>> record = mx.recordio.MXIndexedRecordIO(\'tmp.idx\', \'tmp.rec\', \'r\')
>>> record.read_idx(3)
record_3'

def read_idx(self, idx):

    self.seek(idx)
    return self.read()
