Base variables surface output

[anne-glerum]

This PR provides the option to also output the solution variables on the surface of the mesh.

Discussion in #4054 should be settled to be able to finish this PR.

PR used in #4056 .

For all pull requests:

• [x] I have followed the instructions for indenting my code.

For new features/models or changes of existing features:

• [x] I have tested my new feature locally to ensure it is correct.
• [x] I have created a testcase for the new feature/benchmark in the tests/ directory.
• [ ] I have added a changelog entry in the doc/modules/changes directory that will inform other users of my change.

[anne-glerum]

Hi @bangerth , as discussed in #2982 and this year's hack, I'm reviving this PR. I've rebased and run the two tests locally. I need to update the results of the box test with the tester's results after approval, as the resolution has changed.