valgrind complained here and it is right: in the internal loop of the
assembler we loop over localMatrix.N() and M(). Since dune-functions
uses flat matrices for the localMatrix the number of dof's is dim-times
too large for a powerBasis. In order to spare a double implementation of
the local assembler only for powerBasis, we can simply convert it in the
lambda. valgrind is now happy here.