Add tree "summarize" query mode (#117)

examples/paul_graham_essay/TestEssay.ipynb

gpt_index/indices/base.py

gpt_index/indices/common/__init__.py

gpt_index/indices/common/tree/__init__.py

gpt_index/indices/common/tree/base.py

gpt_index/indices/query/base.py

gpt_index/indices/query/query_map.py

gpt_index/indices/query/schema.py

gpt_index/indices/query/tree/summarize_query.py

gpt_index/indices/response_utils/response.py

gpt_index/indices/tree/base.py

gpt_index/indices/tree/inserter.py

gpt_index/prompts/default_prompts.py

gpt_index/prompts/prompts.py

tests/indices/test_response.py

tests/indices/tree/test_base.py

tests/mock_utils/mock_predict.py

tests/mock_utils/mock_prompts.py