[FEATURE] Added function to fetch products of type simpleProduct

__Tasks:__

- [ ] add new function to fetch products of type simpleProduct
- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | none
| BC Breaks     | none
| Doc           | none
check for $products === null, since this will throw an exception in array_walk