Future of proxsuite-nlp - downstream merger with aligator

[ManifoldFR]

Following internal discussions, we might in the future fold the entirety of proxsuite-nlp into the (currently downstream) aligator numerical OC library - which will from then on be a library for numerical optimization and optimal control in robotics.

This will help make maintenance easier, consolidate the library code (especially coding guidelines and quality), reduce packaging overhead, and make the process of making changes targeted for use in aligator (e.g. changing the manifold or constraint APIs) much easier and smoother.

We hope this change will allow us to iterate faster on both libraries, allow users to contribute features more easily ecosystem-wide.

There are arguments for keeping both libraries separate however.