Replace tovoigh and fromvoight with correct index functions

[TeroFrondelius]

#14 implements stress and strain in tensor format. However, shear stress and strains are in incorrect order, see: Ferrite-FEM/Tensors.jl#122.

According to Tensors.jl documentation tovoight index order is [11, 22, 33, 23, 13, 12, 32, 31, 21] while according to Abaqus Analysis User's Guide: 1.2.2 Convention used for stress and strain components the index order is [11, 22, 33, 12, 13, 23].