❓`parallelization` vs `settings['CMDLINE']` input

[mbercx]

In 4fabecc a parallelization input was added to the BasePwCpInputGenerator as an alternative to specifying parallelization flags (e.g. -npool, -ntg) via settings['CMDLINE']. The latter currently emits an AiidaDeprecationWarning for pw.x and cp.x.

While working on v5.0.0 I considered removing this deprecated path, but held off because:

1. Warning is not seen at submission. The warning is raised inside prepare_for_submission, which is run by the daemon. Users typically never see it when submitting calculations, since it's only present in daemon logs. We should move the deprecation warning to a proper validator before actually removing the possibility of using the CMDLINE.

2. parallelization is only available on pw.x and cp.x. Users of ph.x, pp.x, neb.x, and all NamelistsCalculation-based codes still have to use settings['CMDLINE'] for parallelization. If we want to go for the parallellization input, we should offer it consistently for all CalcJob classes. Also see: extend parallelization concept to PhCalculation #743

We should consider if we want to add parallelization to all CalcJob classes, or remove it entirely in favor of just using a CMDLINE setting. Some benefits for each:

parallelization

• It's easier to find, also clearer in the provenance.
• Straightforward, isolated validation.

CMDLINE

• It's what user know: adding extra command line arguments is simple.
• Manipulating the submission script in AiiDA is typically done via the options. So if we move this flag there (maybe even in AiiDA core), the usage would be more uniform.
• Less code to maintain, especially if we want to add parallelization as an input to all CalcJobs.