Update output view provider interface to support working with intermediate files

From mailing list discussion:

Output view providers, as long as they can work with partial files, should be able to be invoked on these intermediate files as well and so I also need to think about how to integrate those.

 
May need to add a new keyword argument to the generate_data function, intermediate, set to True when the file is an intermediate file. That would enable output view providers to vary their logic based on whether the file is complete or partial.  May also need to add a metadata field to the output view provider class so that output view providers can declare that they don't support intermediate files.