Add V2 version of mnist database

This new version also fixes a bug making in not working on the platform.

advanced/databases/mnist/5.json

+{
+    "description": "The MNIST Database of Handwritten Digits",
+    "root_folder": "/idiap/group/biometric/databases/mnist",
+    "protocols": [
+        {
+            "name": "idiap",
+            "template": "simple_digit_recognition/1",
+            "views": {
+                "train": {
+                    "view": "View",
+                    "parameters": {
+                        "group": "train"
+                    }
+                },
+                "test": {
+                    "view": "View",
+                    "parameters": {
+                        "group": "test"
+                    }
+                }
+            }
+        }
+    ],
+    "schema_version": 2
+}
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