Wrong units of stress tensor

[bosonie]

In the documentation it is indicated that the stress is returned in units of GPa. This is incorrect. The units of the stress are in Ry/Ang³.
The next release will fix the documentation in order to maintain back compatibility.
The release after (2.x), will change the units to ev/Ang³, in order to be consistent with the forces.
IMPORTANT: if units are changed here, then also remember to fix the stress returned in the aiida-common-workflow project!

[bosonie]

Unfortunately I forgot to implement the change of units in 2.0.0. We therefore, at the moment, still have units of stress in Ry/Ang³. If the change wants to be implemented, a deprecation path must be followed and the change introduced in a new major version.