Add package.xml to allow build in catkin workspace

I just spent two full days debugging matrix sizes in Pinocchio that exhausted memory (#275 (closed)) - in the end this was due to compilation issues in configuring different repositories locally on a single machine; every change requires a cascade of reconfiguration & compilation so tracking these things down often leads to going down a rabbit hole.

Adding this package.xml (together with similar changes I am working on) allows building all dependencies as third-party libraries in a catkin workspace. The entire Crocoddyl development workflow with eigenpy, pinocchio, example-robot-data etc. reduces to a single catkin build without any further changes to the buildsystem or CMake files in any of the projects.

As I am the one person onboarding every person at Edinburgh (same for HPP etc. last year) and the only person debugging everybody's problems with the jrl cmake buildsystem at Edinburgh, this will make my life easier and save countless hours.