Publish parser as a generic python library

[alexgarel]

We separated the parser in it's own directory with the idea that we could publish it as a generic module, for it could be useful to other openfoodfacts projects or re-users.

But there are several steps to be done:

Tasks

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1. Separate the parser and the neo4j backend #88
   backend neo4j parser +3
2. Automate parser publication to pypi #89
   GitHub parser +2

Stretch goal:

• Add an example NetworkX backend