Fix bug in random-shortcut.
Fix warning introduced by modification in hpp-model.
Use Eigen::Ref for Jacobian matrices.
Modify Roadmap and Node interface

  - For clarity, define two methods instead of using default parameters:
     Roadmap::addNode,
     Roadmap::nearestNode,
     Node::Node.
  - Computation are unchanged.
Fix bug in DiffusingPlanner when planning with locked degrees of freedom.