temperature method in stokesproblem thows an error, if called an not...

temperature method in stokesproblem thows an error, if called an not implemented. Changed asImp function according to e.g. implicitproblem.hh

Approved by Bernd and reviewed by Markus.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@11940 2fb0f335-1f38-0410-981e-8018bf24f1b0