Move examples to the tutorials repository

[IshaanDesai]

As suggested in #23, it would be appropriate to host the examples in the tutorials repository of the preCICE ecosystem. It is sometimes the case that the examples need to be updated, and that needlessly enforces working in the adapter repository.

Moving the examples to the tutorials repository should be straightforward and does not require a deep understanding of DuMuX, DUNE, or even the examples themselves. In the past, such a move was not done because the examples are used to test the adapter via CI. But if one looks into how the examples are used for testing, it is very close to the system tests framework that we have now. Hence, the tests can be triggered as system tests.