[example] improve Millau example

[Mind-the-Cap]

Changes to the code made by Antoine during his internship:

• Generalised the code, it can now be used for any set of contiguous departments
• Reorganized the code in functions
• Added a similarity function (modified Sørensen similarity index) to match the metrics used in the source research article
• Added functions to find the optimal parameters (correction coefficient, alpha, beta)
• More comments, notably docstrings
• The code now respects PEP8

The global work-home file from INSEE is too big, so I'm not putting in the PR.

It's an ongoing work and many improvements can still be done:

• Make the functions available in the main library and not just like examples
• Improve get_insee_data to incorporate home-work and coordinates files
• Question the outputs of the functions, maybe it could be simple (and anyway the structure of the dataframes should be described)
• Some variable names are not clear or are inconsistent (flowsRM vs flowDT for instance)
• Using global files for France improves easiness of use but increases running time, may be questioned
• Not totally sure it's the best similarity index implementation

[FlxPo]

Thanks for the PR !

I'll keep this discussion in english, as you started (should we stick to english for github ?).

For now I cannot test the code, as some INSEE links are dead (see https://github.com/mobility-team/mobility/issues/22). I'll fix that issue before testing again.

[FlxPo]

I was able to run the code after fixing https://github.com/mobility-team/mobility/issues/22.

I added a function to download the home work flows dataset, because I think all examples and tests in the repo should be able to run easily, without manually downloading some files. What do you think ?

@Mind-the-Cap @louisegontier, can you test if it works for you ? @Mind-the-Cap can you open issues for the possible improvements you listed ?

Link to the commit : https://github.com/mobility-team/mobility/commit/17bc9858d1915a93f2bb3b21999ad62d2166fa4e

[Mind-the-Cap]

The fix works well, and it's a great idea for all examples to run easily (you already ticked one task on my improvement list!)

I will add issues for all other improvements

I used English by habit, but we can definitely use French for PRs and issues (let's stick to English for the code)