Make sts_bt_library a proper class

[kikass13]

THIS ISSUE IS A COPY OF OUR INTERNAL GITLAB ISSUE POSTED HERE FOR DOCUMENTATION PURPOSES

The behavior tree should not be a static class because that inhibits multiple instances (which could result in problems when using a single nodehandle).

• transform the methods into a normal class
• create a singleton pattern around that if necessary (because we want to inhibit multiple instantiations << do we want that?)