Add the possibility of submitting max_concurrent_base_workchains

[bastonero]

The PhononWorkChain can spawn quite a few parallel PwBaseWorkChain, especially in low-symmetry systems. It would be ideal to add the capability to submit only a certain maximum number of them at a time, until the workchain runs out of them.

[cpignedoli]

Hi @bastonero we should discuss the possibility of bringing this to higher priority, we have started having issues with "realistic" systems (~100 atoms), we can discuss this in person in June and later update the issue.

[bastonero]

Hi @cpignedoli , thanks for reminding me this one. It should be actually quite easy, as the logic has already been implemented in aiida-quantumespresso-hp, so we can "simply copy/paste" it here and do some tests to see that everything works fine.