Add parent_folder_ph to PhCalculation

The lack of an extra input for other `PhCalculation`s
makes it hard to devise workflows enabling the parallel
options of `ph.x`. By adding a new input to the CalcJob
different folders from different `ph.x` calculations
be now merged into a unique final folder, the can serve
for instance to collect perturbation files needed for
post-processings or more advanced interpolation
techniques (e.g. via the EPW code).

src/aiida_quantumespresso/calculations/ph.py

@@ -56,6 +56,8 @@ def define(cls, spec):
         spec.input('settings', valid_type=orm.Dict, required=False, help='')
         spec.input('parent_folder', valid_type=orm.RemoteData,
             help='the folder of a completed `PwCalculation`')
+        spec.input_namespace('parent_folder_ph', valid_type=orm.RemoteData,
+            help='the folder of one or more completed `PhCalculation`')
         spec.output('output_parameters', valid_type=orm.Dict)
         spec.default_output_node = 'output_parameters'
 
@@ -299,6 +301,14 @@ def prepare_for_submission(self, folder):
                     os.path.join(parent_folder.get_remote_path(), self._FOLDER_DYNAMICAL_MATRIX), '.'
                 ))
 
+        if 'parent_folder_ph' in self.inputs:
+            for ph_folder in self.inputs.parent_folder_ph.values():
+                remote_copy_list.append((
+                    ph_folder.computer.uuid,
+                    os.path.join(ph_folder.get_remote_path(), self._OUTPUT_SUBFOLDER, '_ph0', f'{self._PREFIX}.phsave'),
+                    os.path.join(self._OUTPUT_SUBFOLDER, '_ph0', f'{self._PREFIX}.phsave'),
+                ))
+
         # Create an `.EXIT` file if `only_initialization` flag in `settings` is set to `True`
         if settings.pop('ONLY_INITIALIZATION', False):
             with folder.open(f'{self._PREFIX}.EXIT', 'w') as handle: