PwBandsWorkChain: default diagonalization set to 'cg'?

[mbercx]

The PwBandsWorkChain automatically sets the diagonalization setting to 'cg' inside the run_bands step of the outline:

aiida-quantumespresso/src/aiida_quantumespresso/workflows/pw/bands.py

Lines 265 to 267 in 35d934e

	# Only set the following parameters if not directly explicitly defined in the inputs
	inputs.pw.parameters['ELECTRONS'].setdefault('diagonalization', 'cg')
	inputs.pw.parameters['ELECTRONS'].setdefault('diago_full_acc', True)

1. I think it would be better if this default is moved to the protocol.
2. We should consider changing this default. It's true that 'cg' is more robust, but also more expensive. There are also newer diagonalization options to consider, see the pw.x documentation.

We'll have to check which diagonalization algorithms have been added since v6.5 (the oldest version we'll support for v4.X of the plugin), and do a bit of testing to see what default would be the most sensible.

[mbercx]

Unfortunately 'paro' has only been implemented since v6.6:

https://gitlab.com/QEF/q-e/-/releases#new-in-v-66