“Supergirl,” which stars Milly Aycock as the titular character, is the latest installment of the big-screen “DC Universe” but feels more like it hails from a galaxy more comparable to the “Star Wars,” “Mad Max,” and “Guardians of the Galaxy” franchises, the latter brought to fruition by this film’s producer James Gunn, currently co-chairman and co-CEO of DC Studios alongside fellow producer Peter Safran. As a result, “Supergirl” struggles — mostly in vain — to establish an identity of its own.