Consider refactoring ProvenanceFileStorage

[sschuberth]

The current ProvenanceFileStorage interface and implementations have some potentially confusing quirks:

• The fun putFile(provenance: KnownProvenance, file: File) function takes a file, but actually only its contents are stored. In particular, the file name is lost and cannot be assumed to be maintained when calling fun getFile(provenance: KnownProvenance): File?. As a result, also the extension of the original file is lost, so one cannot deduce the file type (which implies how its contents are to be interpreted) anymore.
• As another consequence, only a single file can be associated by a given KnownProvenance per ProvenanceFileStorage instance.
• Also, as the interface operates on files, data that is in memory needs to be serialized to disk first before it can be stored, which may be inefficient.

As a straight-forward solution to address this, I'd propose to make the interface operator on Input- / OutputStreams rather than files. Alternatively (but with significant more work), the file name could also be (re-)stored, and multiple files (with different names) per storage could be allowed.