added output control via time interval to decoupled base problem

   - if a time interval is set by the function setOuputTimeInterval(Scalar
     interval) there will be an output at the end of each of these time
     intervals
   - if the output interval (control by output number) is set to 0 (setOutputInterval(int interval)) the
     ouput will only be written after each output time interval



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@8032 2fb0f335-1f38-0410-981e-8018bf24f1b0