[!380] [bugfix] Correctly multiply weights for FastDG, when using accumulate

Merge branch 'bugfix/fastdg-accumulate' into 'master'

ref:pdelab/dune-pdelab In sum factorization code, we are usually doing the accumulation ourselves, so this one went unnoticed.

See merge request pdelab/dune-pdelab!380

(cherry picked from commit 56e0c208)

1451d509 [bugfix] Correctly multiply weights for FastDG, when using accumulate