Possible updates for catalog schema.

Some bug fixes: duplicate platform column and missing target
Addition: dimensions column
Proposal of "required" attribute for each column; either
- true: always required
- enhanced: optional but will be filled by canopy
- false: optional

cats/gfdl_template.json

@@ -3,72 +3,88 @@
   "attributes": [
     {
       "column_name": "activity_id",
-      "vocabulary": ""
+      "vocabulary": "",
+      "required": false
     },
     {
       "column_name": "institution_id",
-      "vocabulary": ""
+      "vocabulary": "",
+      "required": false
     },
     {
       "column_name": "source_id",
-      "vocabulary": ""
+      "vocabulary": "",
+      "required": false
     },
     {
       "column_name": "experiment_id",
-      "vocabulary": ""
+      "vocabulary": "",
+      "required": true
     },
     {
       "column_name": "frequency",
-      "vocabulary": "https://raw.githubusercontent.com/NOAA-GFDL/CMIP6_CVs/master/CMIP6_frequency.json"
+      "vocabulary": "https://raw.githubusercontent.com/NOAA-GFDL/CMIP6_CVs/master/CMIP6_frequency.json",
+      "required": true
     },
     {
       "column_name": "modeling_realm",
-      "vocabulary": ""
+      "vocabulary": "",
+      "required": true
     },
     {
       "column_name": "table_id",
-      "vocabulary": ""
+      "vocabulary": "",
+      "required": false
     },
     {
       "column_name": "member_id",
-      "vocabulary": ""
+      "vocabulary": "",
+      "required": false
     },
     {
       "column_name": "grid_label",
-      "vocabulary": ""
+      "vocabulary": "",
+      "required": false
     },
     {
       "column_name": "variable_id",
-      "vocabulary": ""
+      "vocabulary": "",
+      "required": true
     },
     {
       "column_name": "temporal_subset",
-      "vocabulary": ""
+      "vocabulary": "",
+      "required": true
     },
     {
       "column_name": "chunk_freq",
-      "vocabulary": "https://raw.githubusercontent.com/NOAA-GFDL/CMIP6_CVs/master/GFDL_chunk_freq.json"
-    },
-    {
-      "column_name": "grid_label",
-      "vocabulary": ""
+      "vocabulary": "https://raw.githubusercontent.com/NOAA-GFDL/CMIP6_CVs/master/GFDL_chunk_freq.json",
+      "required": true
     },
     {
       "column_name":"platform",
-      "vocabulary": ""
+      "vocabulary": "",
+      "required": false
     },
     {
-      "column_name": "platform",
-      "vocabulary": ""
+      "column_name":"target",
+      "vocabulary": "",
+      "required": false
     },
     {
      "column_name": "cell_methods",
-      "vocabulary": ""
+      "vocabulary": "",
+      "required": "enhanced"
     },
     {
       "column_name": "path",
-      "vocabulary": ""
-    }
+      "vocabulary": "",
+      "required": true
+    },
+    {
+      "column_name": "dimensions",
+      "vocabulary": "",
+      "required": "enhanced"
   ],
   "assets": {
     "column_name": "path",