Implemente simple GSpline linear operations with SFINAE

[rafaelrojasmiliani]

template <typename T, typename std::enable_if_t<
                          std::is_base_of_v<GSplineBase, T>> * = nullptr>
T operator*(double _a, const T &_that);

template <typename T, typename std::enable_if_t<
                          std::is_base_of_v<GSplineBase, T>> * = nullptr>
T operator*(double _a, T &&_that);