- Use parent implementation for method solve,
  - reimplement tryToConnectInitAndGoals,
  - remove InStatePath class and call core::DiffusingPlanner instead.
  - optimize paths on sub-manifolds with random shortcut and insert path
    in roadmap.

  when calling a path planner in hpp-core.
  Throw core::path_planning_failed when interrupted.

  - methods formerly taking a const reference to Problem as input now take
    a shared pointer to const Problem.