[!385] [bugfix][fastdg] Fix pattern assembly in FastDG

Merge branch 'bugfix/fastdg-pattern-assembly' into 'master'

ref:pdelab/dune-pdelab This fixes a bug that originate from a partial exadune
port. In Exadune, the pattern base classes have a fast version, which was not
ported to PDELab. However, in the absence of these, the PatternEngine of the
FastDGGridOperator needs to be more clever and only operate on one entry of
each block.

See merge request [!385]

  [!385]: gitlab.dune-project.org/pdelab/dune-pdelab/merge_requests/385
2 jobs from master in 14 minutes and 46 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Test
passed #73302
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debian:10--clang

00:14:39

passed #73301
cores:4 mem:16G
debian:10--gcc

00:14:46