Clarified API with respect to DEM tiles overlapping.

core/src/main/java/org/orekit/rugged/api/RuggedMessages.java

...va/org/orekit/rugged/intersection/BasicScanAlgorithm.java

...kit/rugged/intersection/duvenhage/DuvenhageAlgorithm.java

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

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

core/src/main/java/org/orekit/rugged/raster/TileUpdater.java

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

...resources/assets/org/orekit/rugged/RuggedMessages_de.utf8

...resources/assets/org/orekit/rugged/RuggedMessages_en.utf8

...resources/assets/org/orekit/rugged/RuggedMessages_es.utf8

...resources/assets/org/orekit/rugged/RuggedMessages_fr.utf8

...resources/assets/org/orekit/rugged/RuggedMessages_gl.utf8

...resources/assets/org/orekit/rugged/RuggedMessages_it.utf8

...rugged/intersection/duvenhage/DuvenhageAlgorithmTest.java

...rc/test/java/org/orekit/rugged/raster/SimpleTileTest.java