(enh) directly define class label metadata [skip-ci]

Author: rciric

fmriprep/workflows/bold/confounds.py

@@ -245,8 +245,9 @@ def init_bold_confs_wf(
         acompcor.inputs.repetition_time = metadata['RepetitionTime']
 
     # Global and segment regressors
+    signals_class_labels = ["csf", "white_matter", "global_signal"]
     mrg_lbl = pe.Node(niu.Merge(3), name='merge_rois', run_without_submitting=True)
-    signals = pe.Node(SignalExtraction(class_labels=["csf", "white_matter", "global_signal"]),
+    signals = pe.Node(SignalExtraction(class_labels=signals_class_labels),
                       name="signals", mem_gb=mem_gb)
 
     # Arrange confounds
@@ -272,6 +273,8 @@ def init_bold_confs_wf(
         name='acc_metadata_fmt')
     mrg_conf_metadata = pe.Node(niu.Merge(3), name='merge_confound_metadata',
                                 run_without_submitting=True)
+    mrg_conf_metadata.inputs.in3 = {label: {'Method': 'Mean'}
+                                    for label in signals_class_labels}
     mrg_conf_metadata2 = pe.Node(DictMerge(), name='merge_confound_metadata2',
                                  run_without_submitting=True)
 
@@ -397,7 +400,6 @@ def _pick_wm(files):
         (acompcor, acc_metadata_fmt, [('metadata_file', 'in_file')]),
         (tcc_metadata_fmt, mrg_conf_metadata, [('output', 'in1')]),
         (acc_metadata_fmt, mrg_conf_metadata, [('output', 'in2')]),
-        (signals, mrg_conf_metadata, [('metadata', 'in3')]),
         (mrg_conf_metadata, mrg_conf_metadata2, [('out', 'in_dicts')]),
 
         # Expand the model with derivatives, quadratics, and spikes