This issue is to unify the discussion on advanced examples as we were already having in #1101 and possibly other places, too.

The big problem here is that we would like to have two kinds of examples. One which is really introductory and helps people get going with ODL and others which show more advanced functionality of ODL and its usage for "real" problems. It should be obvious to the user which examples are which and it should be easy to navigate in each of these. Thus it probably makes sense to have two different folders for these.