some new functionality of gridadapt class, changed call of

updateMaterialLaw function in base problem

   - GridAdapt: new function wasAdapted() -> returns true or false after
     call of adaptGrid(), new function init() -> calls init() function
     of grid adaption indicator (all indicators need an init() function!), new parameter adaptationInterval -> allows to adapt only at certain time steps,
   - ImpetProblem: updateMaterialLaw() is now called in the
     preTimeStepFunction() per default

   reviewed by Benjamin



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@8403 2fb0f335-1f38-0410-981e-8018bf24f1b0