[add] model for panel data

[MasaAsami]

Thanks for the great list. I would suggest the following three points:

• Synthetic Control Method This is a relatively traditional econometric approach developed by Abadie. I added it because it was frequently used in causal inference in the public policy field.

• Synthetic Difference in Differences This is a recently published paper. As the name suggests, the model is a successful fusion of "Synthetic Control Method " and "Difference in Differences", which gives the impression of being more sophisticated.

• Causal Impact This model is frequently used in marketing-related fields. Since the idea is a natural extension of the Synthetic Control Method, it is included in the same table.

Thank you.

[rguo12]

Thanks a lot for your contribution!