[cleanup] move item generation scripts into subfolder generation; see

notes below

- created __init__.py and __main__.py for generation module
    !! now use python -m ivas_processing_scripts.generation !!
- moved reverb.py wrapper to main wrappers folder
- moved modified config.py to generation/config.py
- moved modified constants.py to generation/constants.py
- moved process_{ism,stereo}_items to generation/
- integrated modifications to audiotools.audiofile.py