adding SummarizingAgent

[ndebard]

This adds a new, fairly simple agent called the SummarizingAgent. It has some interesting features where it can summarize a set of documents (rather than just 1 doc) and does some neat things with chunking up the docs and summarizing in pieces, merging, etc.

• Prompts are added to the prompt library.
• An example is provided in single_agents/summarizing_agent/
• Unit tests are provided

Below is a sample output of running the example, which is itself explained in a README.md file.

[ndebard]

summary.txt

I've attached here a summary example of above, after it summarized those 3 freely available novels. Just in case it's useful to any reviewer. Obviously this stuff is stochastic.