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

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.

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