renamed core package into intersection.

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

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

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

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

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

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

.../rugged/intersection/duvenhage/MinMaxTreeTileFactory.java

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

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

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

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