Modify APIs of RAGChecker

[rudongyu]

Issue #, if available:

Description of changes:

• refactor code for separate modules on data structure, checker, and metrics
• python API support

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[Impavidity]

I think we also need to add the package install for ragchecker. Otherwise, the import ragchecker can not be directly used in other places.

[rudongyu]

I think we also need to add the package install for ragchecker. Otherwise, the import ragchecker can not be directly used in other places.

Sure, will be resolved in another PR.