added load by sql2 function to content mapper

Added a new function, which takes a sql2-query and returns the delivered nodes as structures

- [x] test coverage
- [x] finish the code
- [x] gather feedback for my changes


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | no
| Doc           | none