restructure submodule

[naxingyu]

as discussed in #87

[naxingyu]

test pass, not touching c++, safe to merge. @danpovey

[csukuangfj]

@naxingyu
please have a look at this pullrequest: #88

I reused table.py (https://github.com/mobvoi/kaldi/blob/fangjun-pybind11/src/pybind/util/table.py) from pykaldi.

Current reader/writer in Pybind may be all removed and use
this file https://github.com/mobvoi/kaldi/blob/fangjun-pybind11/src/pybind/util/kaldi_table_pybind.h
instead.

An example for SequentialNnetChainExampleReader can be found at
https://github.com/mobvoi/kaldi/blob/e6610bb666c191f79df327db536f67729fe6c93b/src/pybind/nnet3/nnet_chain_example_pybind.cc#L92-L103

    // (fangjun): we follow the PyKaldi style to prepend a underline before the
    // registered classes and the user in general should not use them directly;
    // instead, they should use the corresponding python classes that are more
    // easier to use.
    pybind_sequential_table_reader<KaldiObjectHolder<PyClass>>(
        m, "_SequentialNnetChainExampleReader");

    pybind_random_access_table_reader<KaldiObjectHolder<PyClass>>(
        m, "_RandomAccessNnetChainExampleReader");

    pybind_table_writer<KaldiObjectHolder<PyClass>>(m,
                                                    "_NnetChainExampleWriter");

https://github.com/mobvoi/kaldi/blob/fangjun-pybind11/src/pybind/util/table.py#L29

and
https://github.com/mobvoi/kaldi/blob/fangjun-pybind11/src/pybind/util/table.py#L158

[csukuangfj]

I think it makes no sense to create a reader without passing a filename and
the default constructor can be simply removed.

[naxingyu]

I see. That's a really huge PR. I can close this one.

[csukuangfj]

I hope that Dan will merge it soon.

We can use SequentialNnetChainExampleReader now in python
and supervision fsts can also be manipulated in Python.

LF-MMI training will soon be available in kaldi pybind.