Use the GlobalDofIndex from dune-parmg

.gitlab-ci.yml: use C++17 for GCC (for `if constexpr`)

See merge request !17

.gitlab-ci.yml: require Python 3

See merge request !19

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.

Remove wrong template parameter (GradientRT) from usages of LocalFEStiffness in film-on-substrate

See merge request !18

To make it clear that that test does not currently test anything.

Previously, the layout of my computer's file system was
hard-coded here.

In this file they are not needed at all anymore.

Film on substrate

See merge request !13

Fix adolc and path

See merge request !15

When using ADOL-C together with MPI we need to use different tags in the
ADOL-C function calls for each process. If we use the same tag, then the
processes will interfere with each other causing undefined behaviour.

I forgot the GridFactory::createGrid did return a plain pointer
as recently as dune-grid 2.6.

This is what GridFactory::createGrid returns anyway.

Add method to compute the local stress of a Cosserat rod

See merge request !11

Adjust riemannian tr solver to upstream changes

See merge request !8