PwBaseWorkChain: Allow restarts from RemoteData that do not have a creator

[mbercx]

Currently, the PwBaseWorkChain expects the RemoteData provided to the pw.parent_folder input to have a creator:

aiida-quantumespresso/aiida_quantumespresso/workflows/pw/base.py

Lines 248 to 249 in c49def7

	if 'parent_folder' in self.ctx.inputs:
	self.ctx.restart_calc = self.ctx.inputs.parent_folder.creator

This is then used to set up a restart_calc in the context, similar to how actual restarts are done in the work chain. However, there can be cases where the user wants to provide a RemoteData that does not have a PwCalculation as a creator. Maybe this is a RemoteData that is constructed manually or is the result of a calcfunction.

Moreover, later in the prepare_process step, the input for the restart_mode tag is overwritten in case a restart_calc is found in the context:

aiida-quantumespresso/aiida_quantumespresso/workflows/pw/base.py

Lines 418 to 420 in c49def7

	if self.ctx.restart_calc:
	self.ctx.inputs.parameters['CONTROL']['restart_mode'] = 'restart'
	self.ctx.inputs.parent_folder = self.ctx.restart_calc.outputs.remote_folder

This means that when providing the parent_folder input, restart_mode is always set to 'restart', overriding the input provided by the user. This means that restarting from only a charge density with startingpot is not possible.