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

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

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.