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...)