Support for Abaqus STRESS and STRAN conversion to Tensor

[TeroFrondelius]

I am making Abaqus UMAT package: https://github.com/JuliaFEM/UMAT.jl and I will need to convert stress and strain tensors. 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]. Could we add methods toabaqus and fromabaqus?