GetTFTransformState tf2 migration

[AravindaDP]

Hi,

I got following exception during a state machine execution which ended up on state machine exiting execution flow due to uncaught exception instead failed outcome at state level. (Note: Looked up transform is an intermittently published TF and failed outcome is the expected outcome when a recent TF publish is not available.)

[INFO] [1597761806.732714, 125.272000]: Getting transform for target = artifact_origin, source = TYPE_HELMET

[ERROR] [1597761824.168765, 129.276000]: InvalidUserCodeError: Could not execute state 'Get_Artifact_Pose' of type '<vigir_flexbe_states.get_tf_transform_state.GetTFTransformState object at 0x7f8e20028e50>': Traceback (most recent call last):
  File "/opt/ros/melodic/lib/python2.7/dist-packages/smach/state_machine.py", line 247, in _update_once
    self._remappings[self._current_label]))
  File "/opt/ros/melodic/lib/python2.7/dist-packages/flexbe_core/core/event_state.py", line 62, in _event_execute
    self.on_enter(*args, **kwargs)
  File "/opt/ros/melodic/lib/python2.7/dist-packages/flexbe_core/state_logger.py", line 105, in event_wrapper
    event_method(*args, **kwargs)
  File "/opt/ros/melodic/lib/python2.7/dist-packages/flexbe_core/state_logger.py", line 167, in on_enter_wrapper
    on_enter_method(userdata)
  File "/home/pramuditha/subt_solution/src/vigir_flexbe_states/src/vigir_flexbe_states/get_tf_transform_state.py", line 63, in on_enter
    listener.waitForTransform(target_frame, source_frame, now, rospy.Duration(4.0))
  File "/opt/ros/melodic/lib/python2.7/dist-packages/tf/listener.py", line 76, in waitForTransform
    raise tf2_ros.TransformException(error_msg or "no such transformation: \"{}\" -> \"{}\"".format(source_frame, target_frame))
TransformException: canTransform: source_frame TYPE_HELMET does not exist.

[ERROR] [1597761824.170464, 129.276000]: Behavior execution failed!

I guess this is due to newer ros versions (melodic in my case) uses tf2 instead tf package. I managed to resolve this by importing tf2_ros package and adding above exception to https://github.com/team-vigir/vigir_behaviors/blob/master/vigir_flexbe_states/src/vigir_flexbe_states/get_tf_transform_state.py#L76.

If it is the correct way to resolve this, I would be more than happy to make a PR for this.