Rivet

Added
- 'exists' function - check if an object exists at a specific S3 key
- Unit tests!

Added
- Specific error for S3 paths that do not include file extensions

Changed
- Hotfix: `*kwargs` changed to `**kwargs` in `s3_write`
- Hotfix: Error string formatting in `get_storage_fn`

Added
- Ability to read files from S3
- Ability to write files to S3
- Ability to list files in S3, with specific prefixes and output settings
- Ability to copy objects between two S3 locations
- Storage formats: CSV, Pickle, Parquet

Changed
- Default CSV writing behavior as was present prior to this code being made into its own package.