Such measurements are now centralized in the program compute-disc-error.cc.

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.

It is still no function in the dune-functions sense, but at least the
basis parameter needs to be a dune-functions basis now, rather than
a deprecated dune-fufem basis.

For those, it is necessary to have separate measurement for the displacement and orientation components.

Also, check whether the input files have the correct length

This creates a field with topological charge 2.

This is a preparation to allow to select the TargetSpace at run-time.

You never know, we may be doing dim!=dimworld grids eventually.

Of course we always need unitvector.hh.  vtkadapter.hh, on the other hand, only
exists in dune-functions 2.4.

This is needed even though we don't actually use MPI here.  Dune aborts with a
run-time error otherwise.

It is so uncompileable that I am not able to fix it quickly.  Since I don't
actually need it right now I let it wait for another day.

And make that code path work again!

That should work when calling the executable from 'build-cmake/src'.

This initial commit simply brings a cleaned-up version of the harmonicmaps program.
The first step will be to make it compute curve-shortening flow on the sphere.

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.

Here is a new program that does nothing but compute discretization errors:
that is L2 and H1 norms of differences between two functions.  The reference
function can be either discrete or given in closed form (the latter is not
enabled yet).

So far I have only used this program to measure discretization errors of
Cosserat shell models, and I did *not* get the error behavior I expected.
So either the error behavior of such shells is weird, or this new measurement
program is still buggy.