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

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

Use the corresponding member method of FieldMatrix.

Because such statements bleed into all the files that include
the headers, and this may lead to compilation errors later.

Cleanup

Cleanup

Cleanup