[DCP-o-matic] Sox Script fake 5.1 from stereo

Hello, I had some feedbacks on the script_upmix : first there was a mistake in method 3 ending in missing right channel or left right inversion. That is fixed. Some think method 2 is quite "loud" : nice for music, concert, musical video clip but less good for subtile dialogs and atmosphere soundtracks. It might be useful. Attached is newer version of the script_upmix (bash script quite linux-universal) Gérald