`HpCalculation`: rename `parent_scf` input to `parent_scf_folder`

[mbercx]

Currently the RemoteData that corresponds to the folder of the pw.x calculation is provided through the parent_scf input. However, this naming gave me the impression that the input had to be the PwCalculation, not the RemoteData folder where it was executed. Since for the PwCalculation we use parent_folder, perhaps we can rename this input to parent_scf_folder?

I'd make a similar suggestion for parent_hp. However, I'm still trying to figure this input out a bit. Why is this a namespace? And why is it a FolderData and not RemoteData?

[sphuber]

a hp.x calculation needs to compute multiple k-points and then perform a post-processing step. The k-points can be parallelized over individual hp.x runs and then a final hp.x calc can be run to perform the post-processing step. To allow this in the HpCalculation plugin, the parent_hp is a namespace that takes the retrieved FolderData nodes of the parallelized calculations. This should ideally also support RemoteData so the data doesn't have to be retrieved. Of course the calculation should then also be updated to not retrieve the files