inclusion in the Astropy affiliated package ecosystem

[Morisset]

I tried to let PyNeb to be an affiliated astropy package. Some important issues avoid this for the moment. The discussion is there:
astropy/astropy.github.com#332
The 2 main blocking pints are:
Documentation:
At the moment, the user manual is almost exclusively in notebook form, and there is no connection between the manual and the API docs in the reference manual. Ideally, when users read the narrative documentation, it would be good to make sure the method/function/class names in the narrative can link to the API docs so that users can find out more about the available options. For consistency with other packages in the astropy ecosystem, you might want to consider using Sphinx as the primary framework for the docs, which would allow you to have API docs (using e.g. sphinx-automodapi) and which also allows notebooks to be used as-is to generate the docs (using nbsphinx). In this way, all the docs would be on a common site rather than be separated into the reference API docs and the user manual. Aside from the framework, the notebooks could do with more explanatory text. It doesn't look like the documentation is automatically built/tested when changes are made to PyNeb. On the plus side, the mailing list is active and user support there is fast.
Testing
The test coverage, as measured by pytest pyneb --cov pyneb, is around 21%, which is very low - it would need to be significantly higher for the package to be accepted. In addition, we would highly recommend setting up a CI service (Travis, AppVeyor, CircleCI, Azure Pipelines, etc.) to run the tests any time a change is made to the repository.

Will try to find time to solve these issues.