Modernize GenEvent

• add initializers to member declarations and make default constructors = default
• add [[nodiscard]] where appropriate
• remove redundant protected declarations
• remove redundant inline declarations
• remove redundant destructors
• prefer STL algorithms
• prefer auto instead of ...::iterator
• remove pragmas specific to Intel compiler
• prefer using over typedef
• remove DataObject inheritance from CrossSectionFSR (which is a class that does not contain any state...)