[ENH] Define channel column for events and Delimiter field for column descriptions

src/common-principles.md

@@ -483,6 +483,7 @@ and a guide for using macros can be found at
         ),
         "Levels": "RECOMMENDED",
         "Units": "RECOMMENDED",
+        "Delimiter": "OPTIONAL",
         "TermURL": "RECOMMENDED",
         "HED": "OPTIONAL",
    }

src/schema/objects/columns.yaml

@@ -56,6 +56,7 @@ channel:
     If multiple channels are specified, separate with commas (`,`) or a
     delimiter specified in the `"Delimiter"` field describing the `channel`
     column.
+  type: string
 color:
   name: color
   display_name: Color label

src/schema/objects/metadata.yaml

@@ -570,6 +570,13 @@ DelayTime:
     This field is mutually exclusive with `"VolumeTiming"`.
   type: number
   unit: s
+Delimiter:
+  name: Delimiter
+  display_name: Delimiter
+  description: |
+    If a column may be interpreted as a list of values, the character that
+    separates one value from the next.
+  type: string
 Density:
   name: Density
   display_name: Density