change DataProcess.output_file to allow multiple

I can't remember if we considered this before, but in creating derived data assets for intermediate analysis results it's becoming clear this would simplify usage quite a bit. In this use case of adding metadata to pre-existing intermediate data, the options are:

- allow a list here
- require reorganizing intermediate data so outputs from a single step share a subfolder (also requires changing any downstream code that relies on the path)
- represent this in the current schema by duplicating the DataProcess object, once for each output

Think we can do a non-breaking update to allow either a single AssetPath or List[AssetPath] ?