Clean: new multigrid

[spossann]

This PR replaces #60

[spossann]

Hi @mateomarin97 - we have done some house cleaning recently on Github and had to move to a new base branch devel-clean. From now on, please continue working on the new branch new-multigrid-clean from this PR. It has exactly the same code as origin/new-multigrid (I merged with devel and pushed).

In case you still have local code on the old branch that you do not want to lose, you can

git fetch
git checkout new-multigrid-clean
git checkout new-multigrid -- .
git commit -m 'add all local changes'