fix: handle get_var_group correctly if the parent group is root

apps/hiopy/_zarr_utils.py

@@ -2,6 +2,9 @@ import zarr
 
 
 def get_var_group(v):
-    store = zarr.open(v.store)
-    parent_group_path = '/'.join(v.path.split('/')[:-1])
-    return store[parent_group_path]
+    z = zarr.open(v.store)
+    parent_group_path = "/".join(v.path.split("/")[:-1])
+    if parent_group_path == "":
+        return z
+    else:
+        return z[parent_group_path]