add a pointer to the root node

Before the offset of the root node was hard-coded.
Now the offset of the pointer to the root node is hard-coded.
That allows us to replace the root node.

pdb-utils/src/main/java/org/lucares/utils/Preconditions.java

@@ -29,4 +29,21 @@ public class Preconditions {
 			throw new IllegalStateException();
 		}
 	}
+
+	/**
+	 * Check that the given values are equal. The check is done with
+	 * {@link Objects#equals(Object, Object)}
+	 *
+	 * @param actual   the actual value
+	 * @param expected the expected value
+	 * @param message  formatted with {@link MessageFormat}
+	 * @param args     arguments for the message
+	 */
+	public static void checkEqual(final Object actual, final Object expected, final String message,
+			final Object... args) {
+		if (!Objects.equals(actual, expected)) {
+			throw new IllegalStateException(
+					MessageFormat.format(message, args) + " Expected: " + actual + " equals " + expected);
+		}
+	}
 }