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 still tested for the dune-fufem wrapper of a first-order basis, while nowadays
the RiemannianTRSolver class is instantiated with dune-functions bases.

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.

This is needed to be able to multiply FieldMatrices with Quaternions.

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.

Needed for gradient-flow applications.

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.

According to Hanne, this is needed to turn 1d harmonic map heat flow into true
curve-shortening flow.

Hardwiring the weighting here is very hacky.  Something more flexible and
general will need to be implemented eventually.

Apparently, nobody had used the evaluateDerivativeLocal method before.

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

This are needed by the gradientflow code to implement the implicit Euler method
for an L2 gradient flow.

This is an initial version; it currently only works for a very limited set of
cases.  I tested it for triangle grids, and that works.

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.