Assert that connected component has at least one node.
Allow to create new configurations as close as desired from an existing node.
Mode displayConfig function to hpp-model.
Fix operator<< for classes Node and Roadmap

  - move operator inside namespaces,
  - operator<< calls method print defined in the classes.
Do not add edges to roadmap while going through the list of connected components

  - adding edges may imply connected component merging and thus modification
    of the list of connected components.
  - store edges to insert in the roadmap and insert them after the loop.