Support for digit sum (Quersumme) and digital root (iterative Quersumme)

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Received user request to add support for "Quersumme", I would then also want to add repeated digit sum, i.e. root sum.

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Note: I should be aware of the English meanings, according to Wikipedia, digital sum corresponds to not being sure whether Quersumme or iterative (oder einstellige) Quersumme is requested.
The one step Quersumme is called digit (without -al) sum.
The iterative Quersumme is called digital root.