add field for "usage" tag on canals (see #348)

data/fields/usage_waterway.json

@@ -0,0 +1,17 @@
+{
+    "key": "usage",
+    "type": "combo",
+    "label": "Usage Type",
+    "strings": {
+        "options": {
+            "transportation": "Transportation",
+            "transmission": "Transmission",
+            "irrigation": "Irrigation",
+            "headrace": "Headrace",
+            "tailrace": "Tailrace",
+            "spillway": "Spillway"
+        }
+    },
+    "autoSuggestions" : false,
+    "customValues" : false
+}