SERIES PREMIERE: This is the newest entry in the Marvel universe of TV shows, and there are enough of those, by now, for a very wide spectrum of expectations and experiences. On the low, disappointing end, there’s ABC’s Marvel’s The Inhumans. On the high, gloriously original end, there’s FX’s Legion. This new series, on Freeform, is destined to fall somewhere in between, but with a slow start and a very high-concept premise. Two teenagers, played by Olivia Holt and