changes in restart behaviour:

- restart mechanism is now called from init(..) method in time manager for all models via problem().restart(...);
- hence, calls to problem or models in application files and in the default start procedure are deleted (included in timeManager)
- timeManager().init(...) now has a real restart bool defining if a restart was called by the user. Bool is true if restart is desired.
- Decoupled models: to enable restart, the writer is only called with the access function to avoid segmentation fault.
- Adapt tutorial to the change

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6737 2fb0f335-1f38-0410-981e-8018bf24f1b0