Modernize WritePackedDst
- prefer Gaudi::Functional::MergingTransformer
- make thread-safe: move datamembers to PackedDst struct
- add 'add(const T&)' members to PackedDst struct for all known types
- replace the big if-then-else-if-then construct with a fold expression over the listed types, which dynamic_casts to the matching type, and then calls one of the provided overloads
Edited by Gerhard Raven