Improve types for `electronic_structure.{core/dos}`

[DanielYang59]

Summary

• Improve types and docstring cleanups for (follow up of #3873):
  • electronic_structure.core
  • electronic_structure.dos
• Nest private classmethod (which should not be classmethod but staticmethod) get_transformation_matrix inside get_moment method 2843854ea848a5a21898593a26acd8dc44059471, rationale:
  • It's used by get_moment alone
  • No unit test added for get_transformation_matrix (usually I would expose such internal function as private method if I need to unit test it)

TODO:

• [ ] Wait until #3873 is merged, this PR is branched on top of #3873