Unify method for adding files to a pass

[aydenp]

Since #4 we have two ways to add files by Data (rather than one already on disk):

• func addFile(named:,from:)
• func addData(named:,from:)

The difference is that the first creates a temporary file on-disk, where the second does it entirely in memory. This is a workaround because the first doesn't support subdirectories (#6).

Ideally, only one of these should exist. Based on the outward appearance of the API, I would vote for the second which doesn't write to disk. This will also be important for #9, where these models may be reused.