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.