Daniel Harding is the definition of a conducting prodigy if ever there was one. Handpicked by Simon Rattle as the next great maestro while still a teenager, Harding’s rise through the normally sluggish world of conducting has been legendary. Now, in an unexpected move, he is coming home to London, there to assume the post of principal guest conductor of the London Symphony Orchestra. Harding had been known to be looking for a full music directorship, but he will conduct a dozen weeks a year with the LSO, and will tour with the ensemble as well, which is nearly as heavy a workload as that assumed by most music directors.