Network calculus

This is the work of @Lucien formalizing Network-Calculus, following @M.Boyer et al. book.

@sophie link with Prosa will come (soon) in a separate merge request

@M.Boyer feel free to criticize the choice of file and theorem names / repartition / documentation (make gallinahtml to build the doc or just ask I'll give you the html pages)

I still have to update the .gitlab file to go through the CI