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.
This method is unnecessary. Calling `setDiameter(startDiameter)` followed by `setDiameter(endDiameter, time)` is equivalent.