Grant Gustin, better known as Sebastian from Glee, has just been cast as Barry Allen aka The Flash in the upcoming season of CW’s Arrow. The plan is for Barry Allen’s character to appear in three episodes of Arrow before The Flash series spins off on its own. Here is the character description from a press release: “Barry Allen (Gustin) is a Central City assistant police forensic investigator who arrives in Starling to look into a series of unexplained robberies that may have a connection to a tragedy in his past. A comic book fan boy, Barry is obsessed …