Replace hard coded class names, add types for io.vasp.sets

[DanielYang59]

Summary

• Improve types for io.vasp.sets and comment tweaks
• Replace hard coded class names
• Use deadline in deprecated decorator for easier tracking of deprecated functions

[DanielYang59]

@janosh Ready for review now, thanks for your time :)

[janosh]

thanks @DanielYang59! nice work! 👍

[DanielYang59]

Thanks for reviewing, and I really appreciate your merging this before splitting vasp.sets :)