Models should be moved from nodes into BehaviorTree

[RichardKelley]

Models are currently placed in Nodes (ex: CausalLMAction), but this means that if two nodes want to use the same model, extra work is required to prevent duplication and share the model.

Centralizing the models at the BehaviorTree level solves this.

Here are the TODOs to make this happen:

• Model configs should be moved into their own files (Model configuration objects should be moved into their own files. #6).
• Model config files should go into a models directory (Need a models directory to store configuration files #7).
• BehaviorTree should be able to store configuration objects (BehaviorTree should be able to store configuration objects #8).
• BehaviorTree should maintain a mapping from model identifiers. This could just be the model name, but may need to include things like the quantization level (BehaviorTree should maintain a map from model IDs to model instances #9).
• BehaviorTree interface should support operations to add_model and maybe remove_model given a config (or name, or object reference...) (BehaviorTree public interface needs methods to add, remove models #10).
• Node interfaces should be updated to take a name or a config for a model, but should forward this to the BehaviorTree for model construction. Nodes should maintain read-only properties that point to their models in the enclosing BehaviorTree (New nodes should pass model names/configs to enclosing BehaviorTree  #11).
• Bump minor version (since we're changing the interface).

Each of these has or will have its own issue for detailed discussion.