Harmonize DataElementValueType and DataElementDataType

[hf-kklein]

• The type name is DataElementDataType
• The name of the attribute having this type is (DataElement)ValueType

Downstream services (namely ahbicht) now need to decide whether to use the name DataType or ValueType. This confusion/ambiguity is unnecessary.

Ideas

• From the DataElements perspective the name "value type" seems more appropriate imo, because it characterizes the data elements value.
• For (also non python) downstream dependents it is easier if we don't change the attribute name but the type name (because this doesn't affect the json serialization)