[Dune] Change in discrete function

[Adrian Burri]

Hello all,

I've changed the template parameter list of the DiscreteFunction: 
instead of the LocalFunctionIterator, that nobody used and nobody 
implemented, the LocalFunction class itself appears as a template 
parameter. I adjusted the derived classes (DFAdapt and DiscFuncArray) as 
well, so I suppose it shouldn't break anything in your code. If it does, 
blame me, shout at me and then we discuss the matter.

Have a nice day

Adi