#255 Remove Plot1DObserver and Data1DObserver

[Ulf Lorenz]

The setup is too cumbersome, especially if you compare it to Python, where plotting just works interactively. Hence, this class makes little sense nowadays, especially because at the end of the day you can only plot one-dimensional functions.

• Remove Plot1DObserver
• remove all references (docs, highlighted demos etc.)
• Replace Data1DObserver by a functor (requires [#196]