Reorganize to more closely match other data interface packages

[ltirrell]

The original motivation for this package, citesdb, assembles the CITES data and releases it as a GitHub package, and then provides means to download it and access metadata.

Another Python data interface interface based on an R package, nhanes, includes data directly in the repo, along with metadata info.

CITES is quite large, so following the approach of citesdb makes sense here as well. Necessary tasks are to:

• create a makefile that handles the creation and GitHub release of a combined CITES CSV file. pycites users could rebuild their own dataset, but the recommended usage will just pull the file from releases and load it in
• add metadata access functions
• include code to access the CITES website directly (like nhanes)