Renamed package dem into raster.

.../main/java/org/orekit/rugged/core/IgnoreDEMAlgorithm.java

...core/src/main/java/org/orekit/rugged/core/RuggedImpl.java

.../org/orekit/rugged/core/duvenhage/DuvenhageAlgorithm.java

...java/org/orekit/rugged/core/duvenhage/MinMaxTreeTile.java

...g/orekit/rugged/core/duvenhage/MinMaxTreeTileFactory.java

.../org/orekit/rugged/core/raster/IntersectionAlgorithm.java

...c/main/java/org/orekit/rugged/core/raster/SimpleTile.java

...java/org/orekit/rugged/core/raster/SimpleTileFactory.java

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

.../main/java/org/orekit/rugged/core/raster/TileFactory.java

...c/main/java/org/orekit/rugged/core/raster/TilesCache.java

...it/rugged/core/raster/CheckedPatternElevationUpdater.java

...t/java/org/orekit/rugged/core/raster/CountingFactory.java

...st/java/org/orekit/rugged/core/raster/SimpleTileTest.java

...st/java/org/orekit/rugged/core/raster/TilesCacheTest.java