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'.

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.

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.

This to clang for pointing out the problem.  I had not noticed it before
because the code was only wrong for dimworld<3, which I never tried.

This makes sequential cosserat-continuum compile again.

Added program to solve a combined minimization problem:
Minmize the sum of an elastic energy (representing the substrate)
and a cosserat energy (representing a thin film on the substrate).

This makes mixed-cosserat-continuum compile again.

But keep compatibility with dune-grid 2.6 as well.