Added a new algorithm of dividend discount model, and improved a document of price_plus_tax.py under the folder "financial""

[Tongcai618]

Describe your change:

• [yes] Add an algorithm?
• [no] Fix a bug or typo in an existing algorithm?
• [yes] Documentation change?

Checklist:

• [yes] I have read CONTRIBUTING.md.
• [yes] This pull request is all my own work -- I have not plagiarized.
• [yes] I know that pull requests will not be merged if they fail the automated tests.
• [ yes] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [yes ] All new Python files are placed inside an existing directory.
• [yes] All filenames are in all lowercase characters with no spaces or dashes.
• [yes ] All functions and variable names follow Python naming conventions.
• [yes] All function parameters and return values are annotated with Python type hints.
• [yes] All functions have doctests that pass the automated testing.
• [yes] All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• [yes] If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[rohan472000]

In PRs description, • [ ] should be replaced by • [x]

Also, build will pass now as #9078 is merged.