Tree Search Implementation

[psaegert]

TODO

• [x] Review & fix implementation
• [x] Manually test implementation
• [x] Adapt unit tests
• [x] Write tests for Conversation and PriorityQueue
• [x] Support softmax sampling of best conversations via meta_temperature parameter
• [x] Support tree with backtracking (new) and without (up to v0.3.1) in LLMCoder.backtracking and PriorityQueue.backtracking, where PriorityQueue.pop() removes "unrelated" branches so that they cannot be backtracked to
• [ ] Store conversation in nested json file (add to 0.4.1)