Fix several minor config issues

src/main/java/com/dfsek/terra/TerraWorld.java

@@ -18,7 +18,7 @@ import java.util.Map;
 public class TerraWorld {
     private static Map<World, TerraWorld> map = new HashMap<>();
     private final TerraBiomeGrid grid;
-    private BiomeZone zone;
+    private final BiomeZone zone;
     private final TerraConfig config;
     private final WorldConfig worldConfig;
     private static final Object lock = new Object();
@@ -70,4 +70,8 @@ public class TerraWorld {
     public BiomeZone getZone() {
         return zone;
     }
+
+    public static void invalidate() {
+        map.clear();
+    }
 }