[will not merge] Backport linear and non-linear examples to deal.II 9.1

[MakisH]

We recently updated added a non-linear example (see #24) and updated the minimum requirements to deal.II 9.2.0. This was good, as we always try to support the latest version of every solver.

However:

• there are many recent systems where getting 9.1 is much easier than 9.2 (see Ubuntu 20.04, which has binaries for 9.1, but currently can only get 9.2 building from source)
• the adapter seems easy to port to 9.1.

This is intended as a separate branch that we can give to users that quickly want to test the adapter/a tutorial, without having to upgrade deal.II. The Pull Request is here to easily see the diff.

[davidscn]

Everything correct here! The only left TODO is to apply the clang-format as provided in the repo here, which is the deal.II standard. At least for consistency, it would be nice. I could do it as well, if you don't have the configuration.

[MakisH]

The only left TODO is to apply the clang-format as provided in the repo here, which is the deal.II standard.

Done, thanks!