Run one SCF calculation for all plugins

[superstar54]

Currently, plugins are independent, meaning one plugin will not know what other plugins are running. Thus, when selecting both Bands and DOS, the app will run two completely independent SCFs in parallel.

This PR adds one SCF step in the main workflow before running any plugins. On the plugin side, I added a requires_scf and from_ctx keys. If any plugin asks a SCF calculation, the main workchain will do that, and save the scf folder as ctx.scf_folder, so that the plugins can use it.

However, this PR is blocked by aiida-quantumespresso, because the scf namespace is required in the PwBandsWorkChain. In principle, one can remove this requirement so that it can use scf calculation from other calculations.
Hi @mbercx , am I correct on this?

[AndresOrtegaGuerrero]

depending on a plugin the settings might be different, perhaps just try to link bands and pdos , but not all plugins.

[mikibonacci]

Hi @superstar54, as we discussed, maybe part of the solution can be to exploit the caching mechanism.
The main advantage is that we can trigger only a previous scf which is really the same. In this way, no errors in connecting things will be done (ex. scf for conventional cell, but then you want the bands for primitive).

However for now all the plugins are submitted at the same time, so no time to create the caching record.

[superstar54]

logic: run a scf if no property is selected and no relax