Modernize TESMerger

• Port to Gaudi::Functional
  • adapt python configurable accordingly, as this implies a change from string properties to DataHandleBase properties
• Make it more generic:
  • use ValueType::clone() if it exists, otherwise use new ValueType( value )
  • automatically switch between Container::insert (in case of eg. KeyedContainer) or Container::push_back (in case of eg. std::vector)
  • if output container type has a reserve(size_type) method, use it
• Add back the instantiation of a merger of KeyedContainer<Track>