Carsten and I decided that in the new interface this method should simply be
operator().  On the other currently some code still relies on it being called
computeEnergy.  In particular, the example programs from the dune-tnnmg paper
do not compile because of this.

This patch introduces the computeEnergy method and makes it behave like operator().
It is deprecated right away to make it clear that it exists only for transition
purposes.