Function addSingleton

CvPlot/ext/catch2/inc/catch.hpp:13362–13364

    ISingleton::~ISingleton() {}

    void addSingleton(ISingleton* singleton ) {
        getSingletons()->push_back( singleton );
    }
    void cleanupSingletons() {
        auto& singletons = getSingletons();
        for( auto singleton : *singletons )