[ProblemSolver] Allow non-graph steering method in createPathProjector

  Path projectors are defined in core::ProblemSolver and overloaded in
  manipulation::ProblemSolver. Formerly, the overloaded method checked that
  the steering method was of type manipulation::SteeringMethod and created
  the path projector with the inner steering of the latter.

  This commit modifies this behavior since calling idl method
  hpp::corbaserver::Problem::createPathPlanner with a core_idl::Problem
  instance as input fails when the problem solver is of type
  manipulation::ProblemSolver.