Virtual Function is deprecated after dune 2.7.

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.

Also adjust CosseratEnergyStiffness and MixedGFEAssembler for the use with a CompositeBasis instead of a PowerBasis

Up until dune-common 2.7, the Dune namespace contained shared_ptr:
First as an actual implementation, later simply as a namespace
import.  Dune-common post-2.7 has removed that import,
and users of shared_ptr really have to explicitly specify
the std:: namespace prefix.

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'.

These two files are very similar, and merging them will reduce
maintenance effort in the future.  As of this patch, there is
now quite a number of preprocessor directives in the new merged
cosserat-continuum.cc.  That is not pretty, but the new state
is still much easier to maintain than two completely separate
files.  What is more, I hope to be able to reduce some of that
preprocessor trickery by smarter use of the dune-functions
basis trees.

This used to work only with dune-fufem bases, but the method
constructBoundaryDofs recently learned to also handle the bases
of dune-functions.

This used to be a yes/no, parameter, now we require 'cube'
rather that 'yes' or 'true'.

The code it contains is adolcnamespaceinjections.hh.

Capturing any polymorphic object by value is wrong,
and gcc 8 started to warn about this in the case
of exceptions.

The latter is deprecated as of dune-functions 2.6.

This is needed to build with a recent dune-common and dune-fufem

gcc warns about some of those nowadays.

Otherwise some dune-common stuff will not work properly,
for example copying of FieldMatrices that use adouble
as their number type.

The method indexSet has been removed from dune-functions
a long time ago.

This is planned to be used to start a fine-grid computation on the coarse-grid
solution, or to start a high-order computation from a low-order one.  This is
helpful, in particular, for measurements of the discretization error, which
take a lot of time.

Previously, the NonplanarCosseratShellEnergy had the normal field of the unit sphere
hard-wired in the code.  With this patch, using general triangulated surface as
reference geometries is possible.

This allows to select the energy at run-time, but this is not what we do.
It also allows to give extra parameters to the nonplanar Cosserat energy,
and that is what we need: in a subsequent patch the main program will compute
a surface normal field and pass it to the Nonplanar Cosserat energy.

We can now completely control the approximation order by a const int,
which is much nicer.

This data is used by the compute-disc-error program to measure discretization errors.

... with values expected to be given in the parameter file.

Scaling the trust region is the key to success, because in general the midsurface
deformation and the rotations will have values in different ranges.