Replaced pixel by cell for Digital Elevation Model elements.

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

...org/orekit/rugged/intersection/IntersectionAlgorithm.java

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

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

.../main/java/org/orekit/rugged/utils/ExtendedEllipsoid.java

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

...kit/rugged/intersection/duvenhage/MinMaxTreeTileTest.java

...java/org/orekit/rugged/raster/CliffsElevationUpdater.java

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

...rg/orekit/rugged/raster/VolcanicConeElevationUpdater.java