Refactor & redesign of the dsl functions for joins

[pSub]

Currently the class names for the dsl are not self-explanatory.

The join with the explicit select function is currently not a beauty, eg.

e1.window (time(30 sec)) join e2.window(time(30 sec)) where (_0 === _0, (x, y) => { (y._1, y._2) })