[!274] Feature/use different dof numbers for cg

Merge branch 'feature/use-different-dof-numbers-for-cg' into 'master'

ref:dominic/dune-perftool I'm not 100% sure this gives the right number of
dofs for parallel applications, but at least for serial applications it is
correct.

See merge request [dominic/dune-perftool!274]

  [dominic/dune-perftool!274]: gitlab.dune-project.org/dominic/dune-perftool/merge_requests/274