Pb: The openrtm architecture loads a shared library to implement the services of
an OpenRTM node. The scheduling properties are ensured by the operating system
running openrtm. Then on the robot, when the shared library is loaded it is
run inside the real-time loop. As sot-hrp-2 was creating the interpreter and
the ros-node in the constructor this used to break HRP-2 real-time OS.

This commit change the initialization of the ros-node to use a multi-threaded
spinner. Then ros calls are handled in a seperate non-realtime thread.

Thanks to Tinne De Laet for notifying.

  Define entity RosTime that exports Ros time in a signal,
  add this entity in class dynamic_graph.ros.ros.Ros.
  Transfer Ros time in signal of type ptime instead of vector.

It happens, especially at init that vector are null (of size 0).
In this case, avoid abrupt failure by just copying the available
data instead of aborting the whole software.

Previous version of abstract-robot-dynamics did not contain the joint
name. Therefore, we had no way to forward the joint names to ROS.

This patch changes the ros_joint_state entity to add a
retrieveJointNames command. This command uses the pool to retrieve an
instance of the Dynamic entity (i.e. robot.dynamic) by using the
entity name. It then iterates on the robot structure to fill the joint
names in the message.

This command is automatically called in the Ros class so this change
is transparent for the end-user.

However, no more assumptions are made on the robot structure so the
sent message is exactly matching the robot structure loaded in the
dynamic entity. In particular, the free floating DOFs are sent and the
hard coded hand values have been removed.

ROS, in fact, requires DOF names and not joint names. Therefore, the
names are computed as follow:
- 0 dof: ignored,
- 1 dof: dof name is the joint name,
- >1 dof: <joint name>_<dof id>

    RosImport entity writes data from a signal to a ROS topic.

    Rename targets
      - dynamic_graph/ros/ros_import/wrap -> ros/ros_import/wrap,
      - dynamic_graph/ros/ros_export/wrap -> ros/ros_export/wrap,
      - dynamic_graph/ros/ros_joint_state/wrap -> ros/ros_joint_state/wrap.
    Add missing linking flags to target interpreter.
    Do not redefine PYTHON_SITELIB, use cmake/python.cmake definition.