Adg

* Update documentation with the paper reference.
* Update contributors' affiliations.
* Fix the symmetry factor for vertex exchange (spotted by mehdi-drissi).
* Fix authors on the LaTeX output file.
* Ensure dependencies to be Python2-compatible (fix broken ReadTheDocs build).
* Fix malfunctioning Travis-CI test suite.

First version published in [Computer Physics Communications](https://doi.org/10.1016/j.cpc.2018.11.023)! 🎉

* Fix affiliations and various typos in documents.
* Add a reference to the paper in the README.
* Fix a test.

- Correct and update the reference to the article to be cited
- Fix the formatting of the DAM affiliation

- Split the option for many-body character in BMBPT between -nobs for the observable and -3NF for the grand potential
- Update the documentation accordingly
- Add GPLv3 license
- Add a test suite for developers