All the Greg Berlanti DC Comics superhero shows for the CW mount a cross-series narrative of some type each season, imitating the common practice, in both DC and Marvel Comics, to encourage sampling of the less popular titles by spreading a continuing plot among several of them. The same technique works on TV, or makes the attempt, as tonight’s Supergirl launches a new mini-miniseries story line, called “Crisis on Infinite Earths,” that eventually will envelop such sister CW series as Batwoman and The Flash, and include, before it’s over, not only those title characters, but Green Arrow, White Canary, the Atom, and even Superman.