Pyneb

Add new test facilities
Update flow
Update calling sequence for AI4Neb to be optional (need to add pn.config.import_AI4Neb() to use ANN)

* Migrate the setup to pyproject.toml. Minimalist version of setup.py is kept.
* Remove the version.py file, the version is now in pyproject.toml
* Move the import ai4neb into a specific config method: pn.config.import_AI4Neb() before using the Machine Learning tools. This avoid annoying message about Tensorflow to appear and speedup the import of pyneb when not using ML facilities.
* Change the name of the package to pyneb instead of PyNeb (seems to be mandatory soon).

Add Calzetti extinction laws

Correct bug in utils/manage_atomic_data/extract_flt : bytes and str confusion.

Change the way the low and High density ratios are obtained. Old way was using the statistical weights ratio, but this only works for S+-type of density diagnostics. Not for IR line ratios.
Add a Notebook in the doc directory to explore Critical densities.

* Add a MonteCarlo_and_ANN notebook to show the use of Neural Network when a lot of data is used.
* manage Poly and CatBoost as method for the getTemDen ANN
* Some refactoring