The interfaces in this file are not pretty at all, and would benefit
from a major overhaul.  This patch changes a few things:

* The code is now in the namespace Dune::Fufem
* The method computeL2Error ist now called computeL2DifferenceSquared,
  and it computes the square of what the old method computed.
* The method computeH1HalfNormDifferenceSquared has been removed.
  It should be replaced by calls to

    computeL2DifferenceSquared(derivative(f1),derivative(f2),quadKey);

I am happy to discuss further interface changes later.