Add more documentation to the hummingbot source code

[james-hummingbot]

Hummingbot is an open source project and we want to welcome collaboration from outside the company.

1. Module level (top of each .py .pyx file) comments explaining the purpose of the file.
2. Function/method level comments.
3. Use pydoc to allow user to output it in a readable format. Think about hosting the output pydoc somewhere. https://docs.python.org/3/library/pydoc.html
4. Use the interrogate package to ensure a certain level of documentation https://interrogate.readthedocs.io/en/latest/

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