- clear() now resets size to 0, sets hasChilds to false, increases modCount by 1
- added method get(double minX, double minY, double maxX, double maxY, Collection<T> values) to get all elements within the specified area.
- added methods get{Min,Max}{Easting,Northing}() to get the extend specified in the constructor.
- made QuadTree.Executor a static inner class, which requires thus again the Generic T (--> influence on other classes using the Executor)
- javadoc improvements
