Add Output and Input types to Transformers

[zurk]

We want to be sure that only compatible Transformers will be linked together.
So it is a good idea to add to Transformer class something like

INPUT_FORMAT = Rdd[Row["cname1", "cname2"]]
OUTPUT_FORMAT = ...

And check compability on linking stage.

@vmarkovtsev please assign me.

[zurk]

On pause for now because there is a more important issue.