Allow HED has an optional column on selected files

Author: VisLab

src/schema/rules/tabular_data/modality_agnostic.yaml

@@ -15,6 +15,7 @@ Participants:
     handedness: recommended
     strain: recommended
     strain_rrid: recommended
+    HED: optional
   index_columns: [participant_id]
   additional_columns: allowed
 
@@ -42,6 +43,7 @@ Scans:
       description_addendum: |
         There MUST be exactly one row for each file.
     acq_time__scans: optional
+    HED: optional
   index_columns: [filename]
   additional_columns: allowed
 
@@ -58,5 +60,6 @@ Sessions:
         There MUST be exactly one row for each session.
     acq_time__sessions: optional
     pathology: recommended
+    HED: optional
   index_columns: [session_id]
   additional_columns: allowed