Add generator data in pspinfo.

It's a proposition of addition into pspinfo to be able to store some data related to the generation of the pseudopotential, if known.

Those storage could be used by ONCV format which may contains the input data used for its generation, but also any other format if this information is known.

That could be an interesting addition for reproducibility.

The generator_input would contain the input data to generate the pseudopotential, and the generator would contain the name of the generating code. About the latter, I'm actually wondering if it's best to propose to store the command line to be used with the input and not just the ocde name…