Standalone BT Builder Service

[jjzapf]

I have created a standalone BT builder service that takes in a domain and problem and computes an action graph and behavior tree. This is useful for testing the BT builder plugins in isolation without needing to create any action executors.

To test this ...

Terminal 1

cd <pddl_directory>
ros2 launch plansys2_executor compute_bt_launch.py domain:=domain.pddl problem:=problem.pddl

Terminal 2

ros2 service call /<namespace>/compute_bt std_srvs/srv/Trigger

You should see the action graph displayed in a "Plan Viewer" window. The plan, dot graph, and BT will also be stored as files in the same directory as the problem.

Additional Changes

I've included a few small changes to STNBTBuilder in this PR as well. The most significant of these is a check in the build_stn function to ensure that we do not create any self-referencing edges.