RResultPtr<Derived> should be convertible to RResultPtr<Base>

For example, this feature would be useful when dealing with TH1D, TH2D and TH3D – users might want to just keep a std::vector<RResultPtr<TH1>>.

std::shared_ptr<Derived> can be cast to std::shared_ptr<Base> via static_pointer_cast. We can either mimic this behavior or make RResultPtr<Derived> explicitly cast to RResultPtr<Base>.