Method optional

CvPlot/ext/catch2/inc/catch.hpp:9190–9193

        }

        auto optional() -> DerivedT & {
            m_optionality = Optionality::Optional;
            return static_cast<DerivedT &>( *this );
        };

        auto required() -> DerivedT & {
            m_optionality = Optionality::Required;