Added factory for tiles.

The factory is to be implemented by the DEM intersection algorithm, at
very low level (typically tiles based on min/max kd-tree for Duvenhage
algorithm).

The tile updater on the other hand is the responsibility of the mission
specific interface.

rugged-core/src/main/java/org/orekit/rugged/core/dem/TileFactory.java

+/* Copyright 2013-2014 CS Systèmes d'Information
+ * Licensed to CS Systèmes d'Information (CS) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * CS licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.orekit.rugged.core.dem;
+
+
+/** Interface representing a factory for raster tile.
+ * @author Luc Maisonobe
+ */
+public interface TileFactory {
+
+    /** Create an empty tile.
+     */
+    Tile createTile();
+
+}