Add Location#isAvailable

See #2196

Author: dualspiral

src/main/java/org/spongepowered/api/world/Location.java

@@ -146,6 +146,16 @@ public Location(E extent, int x, int y, int z) {
         this(extent, new Vector3i(x, y, z));
     }
 
+    /**
+     * Gets if the {@link Extent} this location refers to is still loaded and
+     * available.
+     *
+     * @return {@code true} if this extent is available.
+     */
+    public boolean isAvailable() {
+        return this.extent.get() != null;
+    }
+
     /**
      * Gets the underlying extent.
      *