Silence cppcheck warning.

- Type qualifiers ignored on function return type [-Wignored-qualifiers]:
  Function expecting const int returned int.
Make planning optional. Deprecate reach_time.

- The action message has a new field: skip_planning, used to request for no
  motion planning to take place, case in which the approach time is
  automatically computed from a specified maximum velocity.

- The above point means that the reach_time parameter is no longer required,
  hence has been removed from the action goal message.

- A side-effect of this, is that the reach_time is computed by the approach
  planner, and no longer has to be forwarded all the way down to the
  MoveJointGroup instances.