DDMap.AddGroup() adds groups using the wrong tag number

[mfrischknecht]

This is more of a "lurking bug", since the method itself doesn't seem to be referenced from anywhere else currently; I stumbled upon it simply because I tried to determine what the key value for DDMap.Groups should be.

At DataDictionary/DDMap.cs:24 the DDMap.AddGroup method inserts a DDGrp into the data dictionary definition by using its delimiter field as a key. At the same time, the code at DataDictionary/DataDictionary.cs:594 suggests that the according numbering field should be used (this is what actually gets called in the parsing phase).