Update Use cases variables and datasetfiles authored by Klas, Claus-Peter's avatar Klas, Claus-Peter
## Terminology:
* temporary storage: a DDIFlatDB for importing of dataset files only
* **temporary storage**: a DDIFlatDB for importing of dataset files only
* internal central storage: DDIFlatDB, containing study, questionnaire and variable information in DDI
* external central storage: All published and versioned information's like the DBK/ExploreData currently provides
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