`quantum_espresso`: Fix stress units

[mbercx]

Fixes #262

Currently, the extract_from_trajectory calculation function simply passes the stress from the last frame of the trajectory to the stress output. However, the units of this output array are in GPa, not the agreed upon eV/angstrom^3.

Here we use the pint library to convert the stress array into the correct units.

[mbercx]

I've added the pint library as a dependency, but of course it's already a subdependency through the aiida-pseudo package. So not sure if this is necessary?

[sphuber]

I've added the pint library as a dependency, but of course it's already a subdependency through the aiida-pseudo package. So not sure if this is necessary?

I think it is good practice to add it as a direct dependency since we are directly using their code here as well.

[mbercx]

So if you are confident as well @mbercx I will let you have final responsibility ;)

I better double check it then 😅