Documenting pre-1790 and post-1790 data

[liaochris]

• Pre-1790 data was cleaned by David and Liam. We still need data documentation for the final dataset.
• Post-1790 data for continental debt was cleaned by Chris. We still need data documentation for the continental debt dataset
• At some point we will want to clean the assumed state debt and document the cleaned dataset.

When making the relevant commit, please reference this issue in the commit title.

[liaochris]

@davidch2020 Will your code break if you move the pre-1790 folder from inside cleaning_CD to outside it?

[davidch2020]

I don't remember completely. There's a good chance yes.