Add DF7PN custom fields

custom_field_list.json

@@ -123,5 +123,13 @@
         ["ext_pressure", "divide_by_10"],
         ["mcu_calibration", "none"]
     ]
+},
+"DF7PN": {
+    "comment": "private values see https://github.com/whallmann/RS41HUP_V2",
+    "struct": "<HHxxxx",
+    "fields": [
+        ["flight_number", "none"],
+        ["sonde_type", "none"]
+    ]
 }
 }