Store dependencies

[adrian-golian]

Let's add a constraints/denpendencies file to the repo - perhaps a Pipfile, or requirements.txt.
It would make it easier for people to reproduce.

[adrian-golian]

Example problem:

File "....../dgm_latent_bow/src/seq2seq.py", line 10, in <module>
    import tensorflow.contrib.distributions as tfd
ModuleNotFoundError: No module named 'tensorflow.contrib'

--> The code won't work past TF version 1.13