Api mods

Modifications in the API to match the paper. Mainly Stream operation consume and produce. The examples also changed accordingly.
I did not want to change the vnode. We have two operations that execute a blocking read: read and consume. The first one reads a single dataflow variable. The latter reads an element of a stream and returns not only the value but also the next element of the stream. The optimal way would be to create consume in the vnode, bt I did not want to do it now, thats why I solved it in that way.