Enhancements for `lambda` inputs.

When we switch to C++17 we should use the following model to allow us to pass nullptr into functions for lambda directly. We should also consider switching to std::optional.

template <typename T>
struct remove_cvref {
  typedef std::remove_cv_t<std::remove_reference_t<T>> type;
};
template <typename T>
using remove_cvref_t = typename remove_cvref<T>::type;

template<typename T>
constexpr inline bool is_null(T &&t) {
    if constexpr(std::is_pointer<remove_cvref_t<T>>::value) {
        return std::forward<T>(t) == nullptr;
    } else {
        return false;
    }
}

#define SG_DO_IF_NOT_NULL(lambda) \
  if constexpr (!std::is_null_pointer<remove_cvref_t<decltype(lambda)>>::value) \
    if (!is_null(lambda)) 

template<typename T>
void DoSomething(T &&t) {
    SG_DO_IF_NOT_NULL(t) {
        std:: cout << t[0] << std::endl;
    }
}

int main() {

    auto q = nullptr;
    std::vector<Real> v = {1, 2, 3, 4};
    DoSomething(v);
    DoSomething(q);

    return 0;
}