refactor: update ResonantNucleus type to support both string and array formats

Author: markmikkelsen

src/schema/objects/metadata.yaml

@@ -3045,9 +3045,11 @@ ResonantNucleus:
     For multi-nuclei experiments such as <sup>1</sup>H-[<sup>13</sup>C] MR,
     an array can be used: `["1H", "13C"]`.
     Corresponds to [DICOM Tag 0018, 9100](https://dicomlookup.com/dicomtags/(0018,9100)) `Resonant Nucleus`.
-  type: array
-  items:
-    type: string
+  anyOf:
+    - type: string
+    - type: array
+      items:
+        type: string
 RotationOrder:
   name: RotationOrder
   display_name: RotationOrder