Simon Praetorius' dune-vtk module has much better support for reading
and writing than my old hacks in dune-gfe.  Let's start adopting
dune-vtk for VTK I/O.

In this initial step, dune-vtk becomes a dependency of dune-gfe,
but only an optional one.  This may change.

* Do not test with libc++. As ADOL-C is built using libstd++ we get strange
  linker and run-time errors.

* Mention that the problem with return value of the ADOL-C adouble
  comparison operators has been fixed in version 2.7.2.
  This will allow to eventually remove the code that patches dune-common
  for the CI system.

* Add gcc test with Dune 2.7.

* Remove the clang test with Dune 2.6.  It makes clang crash, but this
  case is not important enough to make me debug it.

foo

Update the image addresses in .gitlab-ci.yml from "duneci/dune:***" to "registry.dune-project.org/docker/ci/dune:***", so the dokcer-images are fetched from the official dune-ci

Python 2's include files still contain the `register` keyword which is no longer allowed in C++17; using it is an error for clang.