Basic arithmetic with two inputs

### User story
As an analytical code developer, I want a function that takes two numbers and either adds, subtracts, multiples or divides them as specified by the user, so that I can practice using Git.

### Definition of done
- [ ] Runs without error
- [ ] Fully meets the user requirements
- [ ] Documented using a [docstring for python](https://peps.python.org/pep-0257/) or [roxygen2 comment for R](https://cran.r-project.org/web/packages/roxygen2/vignettes/roxygen2.html)
- [ ] Peer reviewed and updated if needed

The issue names no file, test, or implementation language. First locate the exercise entry point and determine whether the repository expects Python or R, then check how similar exercises are run. Done means a documented function accepts two numbers, performs the user-selected addition, subtraction, multiplication, or division, and runs without error.

Written by the indexing model from the issue text.