multiview: util function to handle view file path
multiview: compositor output node, to handle multipart better

I will revert (most of) the changes I did to COM_OutputFileMultiLayer
next.
multiview: reverting original changes on File Output Node (multilayer exr)
multiview: compositor File Output Node
cleanup of not needed (inherited) functions
multiview: first step to handle the left and right as individual images
So far it works only with FileOutput Node in composite
multiview: rename use_multipart to use_multiview