It is still part of the official matrix interface, but I doubt
that anybody actually uses it.  I'd rather see blocklevel gone
and replaced by modern C++ introspection.  Let's remove it here
and see who complains.

The dune-common 2.7 release finally adds all those standard
binary addition, multiplication, etc operators for FieldVector
and FieldMatrix.  Hence we do not have to have our own
implementation anymore.  We still keep it for backward-
compatibility with dune-common 2.6, but guard it behind
cpp conditionals.

Previously, they all inherited from LocalGeodesicFEStiffness,
which was weird, because none of them implemented first or
second derivatives as you would expect from something called
'stiffness'.

It's not clear what it is supposed to be testing at all.

And make it actually test something.

Previously the code only tested whether the AverageDistanceAssembler
computed gradient vector and Hesse matrix.  Now the test actually
tests whether these are correct.

This makes the methods AverageDistanceAssembler::assembleHessian
compile again.  And it should be a wee bit faster two, because
matrix symmetry is now exploited better.

Because a Stiffness does provide a first derivative.

LocalFirstOrderModel will be an abstract base class for models that
implement an energy and a first (not no second) derivative.

A LocalGeodesicFEStiffness 'is a' LocalFirstOrderModel, which 'is a'
LocalEnergy.

... rather than a LocalGeodesicFEStiffness.  That is a much
cleaner design -- A 'stiffness' produces first and second
derivatives for an 'energy'.

Currently, all implementations of variational problems inherit
from LocalGeodesicFEStiffness, which requires gradient and Hessian
implementations.  However, most of these implementations really
only implement the energy, and leave computations of first and
second derivatives to algorithmic differentiation.  To make
this clearer, this patch introduces an abstract base class
LocalEnergy for all implementations that really only have
an energy.

Throw out the AmiraMesh support instead.  I have not seen
anybody using Amira for years.

It combined computing rod configurations and their discretization
errors.  Nowadays the way to do that is to use rod3d.cc for the
rod configurations, and compute-disc-errors.cc for the errors.
Hence rod-eoc.cc is not needed anymore and can go.

.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