Add various formats to the http layer to import/export data.
- read routes should at least accept csv output instead of json
- write routes should at least accept csv input instead of json
When other models are referenced (for example a product holdings it's category), it should use the reference field (the human-friendly id) instead of it's raw id.
The format for arrays for embedded data (tariff area prices, composition subgroup contents, places in floors) must be defined.