Merge branch '2019zipper-tiles' into zipper-tiles

src/main/java/org/orekit/rugged/api/RuggedBuilder.java

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

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

.../orekit/rugged/intersection/duvenhage/MinMaxTreeTile.java

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

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

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

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

...org/orekit/rugged/intersection/AbstractAlgorithmTest.java

...rg/orekit/rugged/intersection/BasicScanAlgorithmTest.java

...t/rugged/intersection/ConstantElevationAlgorithmTest.java

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

...a/org/orekit/rugged/raster/DummySRTMelevationUpdater.java

...orekit/rugged/raster/DummySRTMsimpleElevationUpdater.java

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

src/test/java/org/orekit/rugged/raster/TilesCacheTest.java

...ava/org/orekit/rugged/refraction/MultiLayerModelTest.java