streamline Event/EventPacker

• unify function names to allow the code that uses them be more generic
• prefer generic functions over cut-paste-modified code
• use emplace_back the way it is intended
• prefer fmt::format over std::stringstream
• prefer Gaudi::Property
• prefer switch over repeated if
• prefer std::abs over fabs
• convert {Un,}PackPrimaryVertices, {PVToRec,RecToPV}Converter to Gaudi::Functional
• prefer Gaudi::Property over declareProperty
• prefer range-based loops
• prefer fmt::format over std::ostringstream
• rename select_subrange into subrange
• prefer nullptr over 0

must be applied in conjunction with Moore!955 (merged)