Array improvements for mapper

Needed abilities for content form generator:

- [x] Array for value only
- [x] Add Element
- [x] Remove Element

Checklist:

- [x] finish the code
- [x] gather feedback for my changes
- [x] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | none
| Doc           | [url to doc|none]