added the possibility for restricting the size of a new time step in

impet models

   - The default value for the Impet.DtVariationRestrictionFactor is
     such that the time-step variation is not restricted. Values between
     zero and one restrict in both directions, values greater than one
     only restrict larger time steps
   - reviewed by Bernd



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@10691 2fb0f335-1f38-0410-981e-8018bf24f1b0