Add Causal Inference by H&R data

[malcolmbarrett]

This PR does a few things:

• Adds the Causal Inference: What If? data + documentation
• Fixes the tibble import (these were already tibbles, as are yours, but for that to work generally, you need to import tibble + use @importFrom.
• Adds me as an author

This PR only modifies the R package! I don't know how the python or Stata setups work, so it's probably more efficient for you to make the corresponding changes

[NickCH-K]

Thank you!