Add Surrogate Index module

[paullo0106]

@huigangchen introduced The Surrogate Index: Combining Short-Term Proxies to Estimate Long-Term Treatment Effects More Rapidly and Precisely and it's now on CausalML's roadmap. I'm creating a discussion thread to brainstorm further and track the development.

• Paper study group: hosted and done by @huigangchen (internal doc here)
• Resource collection - read the code repo (in Stata) by JamesStratton
• Create python notebook prototype for Treatment effect estimation (Part A & B) - done by @huigangchen
• Create python notebook prototype for Treatment effect estimation (Part C)
• Create python notebook prototype for Treatment effect estimation (Part D)
• Create python notebook prototype for Treatment effect estimation (Part E)
• Model and interface design (e.g. code structure, function input and output, and etc.)
• Model implementation
• Unit test
• Docs
• Notebook example

[paullo0106]

@huigangchen @ppstacy @vincewu51 Please feel free to add/ edit!

[ludovico-lanni]

Hello! Any news on this implementation plan?

[jeongyoonlee]

@huigangchen, Can you share the prototype notebook you implemented here?