Behavior Tree Compilator PoC: Blockly -> PDDL

This work is focused on building a minimalistic environment, where we develop a behavior tree using the blockly visual framework and then produce (compile) the tree structure into python code that builds and executes the tree. We can break down the work into several steps:

1. Fork pddl-blockly PoC into bt-blockly PoC as a blueprint to start from. [Directory fork done](https://github.com/AI-Planning/blockly-pddl/tree/main/poc-blockly-bt).

2. Explore how Blocks are defined in the [BT-PoC](https://github.com/AI-Planning/blockly-pddl/blob/main/poc-blockly-bt/blocks/operational_model_behavior_tree_PoC.xml), the file can be loaded in the [blockly factory](https://blockly-demo.appspot.com/static/demos/blockfactory/index.html).

3. Update poc-blockly-bt with several sample behavior tree nodes - sequence, control, parallel and a leaf-node.

4. Update the compilation of the blockly tree into [Py Trees framework](https://py-trees.readthedocs.io/en/devel/), similarly to the functionality of "Show PDDL". The construction can be simplified - e.g. first generating all the behavior nodes (using default classes in Py Trees), then generating the code that plugs them together, producing the final tree at the output of the compilation.