Add javadoc for "zipper tile" to deal with non overlapping DEM (WIP)

Algorithm to be defined for DEM tiles without the same resolution.
Some tests around anti-meridian need to be added.
Some tests with more real data to be added.

src/main/java/org/orekit/rugged/raster/EarthHemisphere.java

@@ -7,6 +7,7 @@ package org.orekit.rugged.raster;
  * <p>
  * For Longitude: WESTEXTREME / WEST / EAST / EASTEXTREME
  * @author Guylaine Prat
+ * @since X.x
  */
 public enum EarthHemisphere {
 

src/main/java/org/orekit/rugged/raster/Tile.java

@@ -289,6 +289,8 @@ public interface Tile extends UpdatableTile {
      */
     Location getLocation(double latitude, double longitude);
 
+    
+    // TODO GP check if useful ...
     /** Get tile neighborhood (in case the DEM tiles are not overlapping).
      * <p>
      * TBN: used to create zipper tiles in case of DEM with tiles not overlapping.