Write: String should always be a tuple of (id, content)

[TimDaub]

• in both the extractor and transformer interface we currently accept Write: String and write it subsequently to flat files
• but we have big problems joining results later merging results of two different strategies is painful and all past workarounds have lead to bad code structure #236
• hence, it'd be great if for every submitted write, we'd return a tuple of the content's ID and the content
• this can be a first step towards improved consistency and Choose an embedded database that fits the "load" strategy use case of neume #246