Retriv

Added
- Added `advanced_retriever.py`.

Changed
- Text preprocessing refactoring.

Added
- Added doc strings to `sparse_retriver.py`.
- Added doc strings to `dense_retriver.py`.
- Added doc strings to `hybrid_retriver.py`.

Added
- Added `sparse_retriver.py`.
- Added `dense_retriver.py`.
- Added `hybrid_retriver.py`.
- Added `encoder.py`.
- Added `ann_searcher.py`.
- Added `merger.py`.

Changed
- Almost everything.

Removed
- Removed dependance from `cyhunspell`.

Changed
- Search efficiency improvements.