Use to_quat in lanelet2_global_planner to convert heading to quaternion representation

Description

This MR closes #755 (closed). This changes the quaternion calculation to in the Lanelet2GlobalPlannerNode::current_pose_cb to use the new to_quat method in the motion_common library rather than the current more computationally expensive calculation process. This should also fix the issue that was reported in #755 (closed) regarding incorrect quaternion calculations.

Notes for reviewer

To test this:

1. Run the tests to make sure that no tests fail due to the changes
2. Launch the AVP demo using the following instructions: https://autowarefoundation.gitlab.io/autoware.auto/AutowareAuto/avpdemo.html

It is a little hard to tell the different just from reviewing the demo so I have inserted what I believe to be the defining difference between the thye two implementations:

[lanelet2_global_planner_node_exe-14] [ERROR] [1614320126.688395031] [planning.lanelet2_global_planner_node]: new method
[lanelet2_global_planner_node_exe-14] [ERROR] [1614320126.688454812] [planning.lanelet2_global_planner_node]: x:0 y:0 z:0.993107 w:-0.11721
[lanelet2_global_planner_node_exe-14] [ERROR] [1614320126.688475193] [planning.lanelet2_global_planner_node]: old method
[lanelet2_global_planner_node_exe-14] [ERROR] [1614320126.688486651] [planning.lanelet2_global_planner_node]: x:0 y:0 z:-0.993107 w:0.11721

The above debugoutput shows the new method (this MR) vs. the existing method. There is a clear difference in the signs of the z and w components of the quaternion.

Additionally, I noticed that in the original method for to_quat the decltype casts were swapped. While this doesn't have any effect on the outcome I made a fix to mitigate any confusion.

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