WIP: Use functions for bulk manipulation with attributes to optimize I/O

Use functions for bulk manipulation with attributes to optimize I/O

Functions as e.g.

void GenEvent::add_attributes(const std::string& name, const std::vector<std::pair<int, std::shared_ptr<Attribute> > > &atts)

would allow to add attributes in bulk without multiple locks. The functions should be used in Readers (To be implemented).