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. 
Actually that option is there, as a dataset property named "base.path". In this case, we do not use that option and let the File implementation generate a base path. I can adapt this example by setting that path explicitly for one of the two datasets, to test that, too.