This broke the usual "make; make install" workflow resulting in strange
errors.

Reportedly, norms are always non-negative anyway...

If 'adaptive' is chosen, the program produces the grids shown in the paper
as an illustration of how adaptive nonconforming refinement can lead to
holes (Figure 5, currently).

In particular, this code is used to create some of the figures in the
FoamGrid paper.

FoamGrid is a simplex grid, so AffineGeometry is sufficient.  MultiLinearGeometry
also works, but is more complicated than necessary.

Starting from a regular icosahedron the grid can be refined towards the shape of a sphere.
The example solves an h-adaptive finite volume method explicit Euler scheme for the heat equation.

shows the different possibilities of growth on a simple grid.

Add and implement the methods preGrow/grow/postGrow
Add mark methods for each type of growth (the mark doesn't interfere with the adaptivity interface)
Add a getGrowthMark method to retrieve growth marks independently from adaptivity marks
[backward-compatibility] with dune 2.3

- return mightVanish also if the element is marked for removal
- for isLeaf all children have to be checked as some might have been removed on the leaf

- A FieldVector to store the growth point if the element spawns a new element
- a facetIndex: the facet that the growth is executed on
- a pointer to a facet of another element that we want to merge with
- a boolean that blocks coarsening of this element if one facet is on a bifurcation without father

VANISH the element is going to vanish during the growth step
ADD_NEIGHBOR the element is going to produce a new neighbor element on one of its facets
MERGE_WITH_NEIGHBOR the element is going insert an element in order to connect one
of its facets with a neighbors facet

[backward-compatibility] Use the correct unit test header file names, depending on the dune-grid version

A triangle shaped one-dimensional network in a two dimensional world is
constructed where the elements are parametrized to map onto the unit circle.
Refinement shows that the grid gets closer to the unit cirlce with every refinement.

Then we solve a 1d heat equation using finite volume scheme on the finest circle grid