To build these combinations with the CI-system, install foamgrid for the tests with parmg, vtk, curvedgeometry and curvedgrid

The modules dune-fufem and dune-matrix-vector
have been moved from the FU Berlin server to the server
operated by the Dune project when Carsten Gräser left
FU Berlin.

2.7 is simply too old now.  We could even go straight for 2.9,
but let's try how far we get with 2.8.  That one is in current
Ubuntu and Debian releases.

Add dune-curvedgrid in .gitlab-ci.yml to the ci-runners that test the options with dune-curvedgeometry

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.