Simplify aiida notebook through one-shot WG creation

[GeigerJ2]

We previously first created all the tasks without links and then added the links "manually" at the end. This was done to obtain the same order of tasks as when exporting from the jobflow notebooks—for side-by-side comparison and debugging during development.

However, this was overly complicated and is different from how one would typically construct a WorkGraph, where one can directly pass the output socket of the first task to the input of the second task when creating the second task. This simplifies the creation of the EOS WG, as, for example, we don't require two for-loops over the strains anymore (previously, one was used to add the tasks, the other one to add the links once all the tasks had been added in the desired order).

The various pickle_<something>_tasks are still a bit annoying, as one would usually just pass these values as inputs when running/submitting the WG, but we need them to obtain input data nodes in the exported universal representation.