Deprecate positional field

Another last minute change, I would like to stuff into 2.4 is the deprecation of the positional field parameter. If we want to move into the direction of #385 (closed) and ultimativly, something like #368 (closed), we need to make the breaking to change and make field optional. I think now would be a good time to introduce the upcoming change. The actual Code change is rather small, but a lot of tests and documentation needs to be adjusted. However, I think, this is a worthwile effort. What do you think @palmb and @luenensc?