Otherwise, the ParaView 'glyph' filter won't recognize the direction fields.

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.