resolves problem with non const pointer, but the modification of the
DuneExpressionToCExpressionMapper is still there, maybe even worse now

# Conflicts:
#	python/dune/perftool/blockstructured/accumulation.py
#	python/dune/perftool/blockstructured/argument.py
#	python/dune/perftool/blockstructured/vectorization.py

(cherry picked from commit 822ce8e3)

(cherry picked from commit 0f8daff7)

i.e. this leads to:
    double* x_vec = &x(lfsv,0)
only works for Qk elements!
Problem: the coefficient is const, but I don't know how (if) loopy can generate a const double*