added missing calls for *.clear() in reset method

   - reviewed by Klaus

Dumux-Svn-Revison: 9957
Ported-By: Andreas Lauser <andreas.lauser@iws.uni-stuttgart.de>
changed reset of CFL-Timestep-Estimator

   - reset is now done directly in the transport loop to
     avoid errors

   -Reviewed by Klaus

Dumux-Svn-Revison: 9958
Ported-By: Andreas Lauser <andreas.lauser@iws.uni-stuttgart.de>
corrected and improved implementation of the coats-impes-stability criterion

   - Partly reviewed by Klaus, will be further reviewed by Bernd

Dumux-Svn-Revison: 9959
Ported-By: Andreas Lauser <andreas.lauser@iws.uni-stuttgart.de>
update reference solutions of the decoupled tests after the changes in the time stepping