Add Population Date and Population Source fields (#261)

* Added Population Date and Population Source fields

* drop icon

Co-authored-by: Martin Raifer <martin@raifer.tech>

data/fields/source/population.json

@@ -0,0 +1,14 @@
+{
+    "key": "source:population",
+    "type": "combo",
+    "label": "Population Source",
+    "prerequisiteTag": {
+        "key": "population"
+    },
+    "snake_case": false,
+    "caseSensitive": true,
+    "terms": [
+        "census",
+        "population reference"
+    ]
+}