Caroline Framke Chief TV CriticDo you have to have watched the History Channel’s “Vikings” series in order to fully appreciate “Vikings: Valhalla”? For this newcomer, at least, the answer is…well, yes and no.The eight-episode season of “Valhalla,” which dropped on Netflix on Feb. 25, takes place 125 years after the events of the original “Vikings” series, with Christianity taking over Norse traditions and England’s power on the rise.