Change boundary nature reserve / national park areas

Render nature reserve / national park areas with a double uninterruped line,
like marinas are currently rendered.

* Resolves #69
* Resolves #563

admin.mss

@@ -92,16 +92,23 @@
       opacity: 0.05;
       polygon-fill: green;
     }
-    ::line {
-      opacity: 0.15;
-      line-color: green;
-      line-width: 1.5;
-      line-dasharray: 4,2;
-      [zoom >= 10] {
-        line-width: 3;
-        line-dasharray: 6,2;
-        line-join: bevel;
-      }
+    a/line-width: 1;
+    a/line-offset: -0.5;
+    a/line-color: green;
+    a/line-opacity: 0.15;
+    a/line-join: round;
+    a/line-cap: round;
+    b/line-width: 3;
+    b/line-offset: -1.5;
+    b/line-color: green;
+    b/line-opacity: 0.15;
+    b/line-join: round;
+    b/line-cap: round;
+    [zoom >= 14] {
+      a/line-width: 2;
+      a/line-offset: -1;
+      b/line-width: 6;
+      b/line-offset: -3;
     }
   }
 }