API: Use unique_ptrs for Behavior Nodes

Currently the Behavior Tree is built up with shared_ptrs. However unique_ptrs would express the original intent of a tree.

Outcome of discussion with Ben Wassermann & @rparisha2 --> Change it to unique_ptrs

UPDATE: In PR https://gitlab.eclipse.org/eclipse/simopenpass/yase/-/merge_requests/4 the Services, Conditions etc are already made unique