Implement DoFAccessor::set/get_mg_dof_indices for 1D

[peterrum]

This PR is part of the effort to introduce the new parallel::fullydistributed::Triangulation (see #8558) and is a follow-up to PR #8592.

This PR implements DoFAccessor::set_mg_dof_indices and DoFAccessor::get_mg_dof_indices for 1D. We need this, since p:f:t works for 2D, 3D, and 1D(!) for GMG.

Note: Please only consider the last commit, since all other commits will be removed from this PR once PR #8592 is merged. I still have to add tests.

Ping: @bangerth @tjhei

[bangerth]

The last commit is ok. If you do

  git checkout dofaccessor_mgdofindices_1D
  git rebase -i master

and then drop all of the commits except the last one, you will end up with a branch that has only this last commit on it. That's the one we want to review and merge. (And of course this has no influence on all of your other branches: They continue to contain all of the commits you had on there before!)

[peterrum]

@bangerth I have updated all PRs so that each of them has exactly one PR.

[bangerth]

Thanks, that is much better!

[masterleinad]

/rebuild