Store deformation gradient and stress as Voigt vectors

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.