Feature/simplify problem setup

See merge request dune-microstructure!6

Feature/unit test

See merge request dune-microstructure!5

Various cleanup and speedup patches

See merge request dune-microstructure!4

Voigt-Notation distinguishes in the transformation from Matrix to Vector between stresses and strains.
The transformation for strains features an additional factor 2 for the non-diagonal entries. In order to avoid
the use of different data structures for both stresses & strains we use the same Matrix-to-Vector
mapping ('matrixToVoigt') and incorporate the factors in suitable places. namely:
  - The Stiffness matrix of the constitutive relation gets scaled by a factor of 2 in the last three columns
  - The 'voigtScalarProduct' scales the last three products by a factor of 2

Since all element geometries are identical, the element stiffness
matrices only depend on the phases at the quadrature nodes.
We use those to access a cache.  This leads to tremendous time savings.

This will be the cache key.

That's the first step towards caching the element matrices.

That's cheaper than first computing the symmetric part as a matrix,
and then converting that to Voigt notation.

This again saves a few conversions.

This avoids many transformations from symmetric matrices to
Voigt vectors and back.  In my (limited) testing, this reduces
the time to assemble the stiffness matrix by about 25%.

This patch also introduces a custom scalar product method for
Voigt vectors, which reproduces the Frobenius scalar product
in matrix space.  That way, the potentially confusing distinction
between stress-like Voigt vectors and strain-like Voigt vectors
can be avoided.

There is no need for this.  At least, I don't see any.

Yes, it is debatable whether that change makes the code more readable.

Use the corresponding member method of FieldMatrix.

That's Dune convention.