Decouple StreamGraph Building from the API

Currently, the building of the StreamGraph is very intertwined with the API methods. DataStream knows about the StreamGraph and keeps track of splitting, selected names, unions and so on. This leads to the problem that is is very hard to understand how the StreamGraph is built because the code that does it is all over the place. This also makes it hard to extend/change parts of the Streaming system.

I propose to introduce "Transformations". A transformation hold information about one operation: The input streams, types, names, operator and so on. An API method creates a transformation instead of fiddling with the StreamGraph directly. A new component, the StreamGraphGenerator creates a StreamGraph from the tree of transformations that result from program specification using the API methods. This would relieve DataStream from knowing about the StreamGraph and makes unions, splitting, selection visible transformations instead of being scattered across the different API classes as fields.