Further work on new decoupled structure:

- primary variables vector removed from variableclass -> to pressure / transport model
- Problem applies old time Integration method due to compatibility with compositional models
- Explicit transport is done in transport model
- simplified serialization/restart mechanism
Additonal improvements:
- reordered iteration procedure in impet to avoid superfluous creation of large vectors only used for iteration
- cleaned up serialization procedure

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7342 2fb0f335-1f38-0410-981e-8018bf24f1b0