Reintroduce GraphExtension as a temporary fix

Author: Sascha Fendrich

core/src/main/java/com/graphhopper/storage/GraphExtension.java

@@ -0,0 +1,8 @@
+package com.graphhopper.storage;
+
+// TODO ORS: this is a transitional class which should be eliminated in the long run
+// ORS-GH MOD - additional class
+public interface GraphExtension extends Storable<GraphExtension> {
+    default long getCapacity() { return 0; }
+}
+// ORS-GH MOD