[FEATURE] Extended product with unit and order related properties

__Tasks:__
- [x] add properties to product
- [x] got feedback

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | none
| Doc           | none

Can you not do `setContentUnit(Unit $unit = null)` ?