Fix ValueError while writing atoms with stress tensor to extxyz

I just came across an ase.io.write failure and realized that it is because a caclculator have 3x3 stress tensor and the extxyz writer supposes voigt format stress.

I'm not sure this is a bug but I think it is useful if the writer explicitly uses voigt format stress.

Closes #783

Checklist

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Issue is resolved via "closes #XXXX" if applicable.