[DDI-SRG] [MRT] Model cleanup: string definitions

[Wackerow, Joachim]

The current approach of string types seems to be too complicated. The structured string types could be folded into the regular ones. Furthermore, there are some inconsistencies.
Diagrams on the current approach and the suggested approach are attached. Side note: XSD data type "date" is now used instead of a special DDI data type for date.

Please review the proposal and comment.

Achim

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