WorldBorder addition and changed to clear confusion

Added new method to set the startingDiameter, endDiameter and the time it will take [void setDiameter(double startDiameter, double endDiameter, long time);]
Changed Radius to Diameter.
Changed getCenter to Vector2d as there is no y-axis involved.
Changed get/setBlockBuffer to get/setBuffer and changed the return and parameter to double as it is not measured in whole blocks.
The method is there to also help people so they don't have to call 2 methods to create a world border. instead they only have to invoke 1.