create financial/price_plus_tax function

[alirezasariri78]

Pull Request Template

Description

Please include a summary of the change and which issue (if any) is fixed.
A brief description of the algorithm and your implementation method can be helpful too. If the implemented method/algorithm is not so
well-known, it would be helpful to add a link to an article explaining it with more details.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I ran bellow commands using the latest version of rust nightly.
• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• I ran cargo fmt just before my last commit.
• I ran cargo test just before my last commit and all tests passed.
• I added my algorithm to the corresponding mod.rs file within its own folder, and in any parent folder(s).
• I added my algorithm to DIRECTORY.md with the correct link.
• I checked COUNTRIBUTING.md and my code follows its guidelines.

Please make sure that if there is a test that takes too long to run ( > 300ms), you #[ignore] that or
try to optimize your code or make the test easier to run. We have this rule because we have hundreds of
tests to run; If each one of them took 300ms, we would have to wait for a long time.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.44%. Comparing base (5947a3f) to head (fb7ae17).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #829   +/-   ##
=======================================
  Coverage   95.43%   95.44%           
=======================================
  Files         315      316    +1     
  Lines       22808    22839   +31     
=======================================
+ Hits        21767    21798   +31     
  Misses       1041     1041           

☔ View full report in Codecov by Sentry.
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[alirezasariri78]

changes are done as you said :)

[vil02]

@alirezasariri78: please make sure to update your branch - there are some new warnings in clippy and without the update the CI will fail (cf. #838).

[alirezasariri78]

document added to module file