Feature/file ds

- adds a File dataset interface, also Input/OutputFormatProvider 
- an implementation named FileDataset, including a definition and admin
- Map/reduce runner to support Input/OutputFormatProvider in addition to BatchReadable/Writable
- utilities for scoped runtime arguments ("dataset.<name>.my.property"). 
- unit tests

Please give feedback especially about the APIs. 
This allows people to read or write files when they are not using MapReduce. For example, a spark job computes a model and saves it to a file. A flow loads the file into memory. For that kind of interaction, direct access to the filesystem is needed. And since Location is our file system abstraction, that is used here.