Add an ExoDataSource for CEPII-BACI dataset

[khaeru]

This data set is publicly available, so we can use the preferred pattern to:

1. Fetch it from its publication, with verification and hashing.
2. Unpack.
3. Provide filtering and caching using the existing features of ExoDataSource.

This data is used in .tools.bilateralize; see #438.

Documentation for the data source/set is at:

• https://www.cepii.fr/DATA_DOWNLOAD/baci/doc/baci_webpage.html
• https://www.cepii.fr/CEPII/en/bdd_modele/bdd_modele_item.asp?id=37