[Dune] Deprecating begin() and end() in IndexSet

[Robert Kloefkorn]

Hi Oli,

I'm not sure whether I remember correctly, but when deprecating a 
template method, the implementation has to be outside of the class.
Still a bug, but maybe this way it works.

Cheers,

R

-- 

   Robert Klöfkorn           <robertk at mathematik.uni-freiburg.de>

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