Repeat changes to impetproblem.hh to onemodelproblem:

- the writer is only called with the access function to avoid segmentation fault.
- Added restart function

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6738 2fb0f335-1f38-0410-981e-8018bf24f1b0