Add type hints and doc strings

[aj3sh]

Use Case
This Python package doesn't contain proper type hints and doc strings. A proper type hint and doc string help the end user to use the package. Furthermore, it will also help generate the docs through the codes.

Challenges
As this task involves all the files within the repository. The chances of occurring merge conflicts are high.

Proposed Solution
Partial changes with partial PR will help reduce the merge conflicts. That is to say, only one file will be refactored at once. If anyone is picking to refactor a file, Please comment on this PR.