Surely writer-director Joss Whedon found himself thinking about “Batman & Robin” and similar efforts when charged with bringing the Marvel Comics superhero group The Avengers to the screen. Success would mean merging characters from several previous Marvel movies – “Iron Man,” “Captain America,” “Thor” and “The Incredible Hulk” – and introducing them to even more players. The potential for failure was immense, making it rather astonishing that Whedon not only succeeded but produced one of the best films of 2012 so far.