[functions.checkerboard] major rewrite * adation to new interface * there was a major problem when a discontinuity was   on an intersection. Evaluationg on the intersection   lead to undefined behaviour. So now we assign the value   of the local function according to the center of its   entity only. This leads to a locally constant function,   so the grid better resolves the discontinuity!
[functions.checkerboard] major rewrite
* adation to new interface
* there was a major problem when a discontinuity was
  on an intersection. Evaluationg on the intersection
  lead to undefined behaviour. So now we assign the value
  of the local function according to the center of its
  entity only. This leads to a locally constant function,
  so the grid better resolves the discontinuity!

renemilk/DUNE-Stuff,renemilk/DUNE-Stuff,renemilk/DUNE-Stuff