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.
A `Vector3d` was used to avoid confusion. World borders extend infinitely on the y-axis, and thus only have meaningful x and z coordinates. Using a `Vector2d` would cause the z coordinate to be stored in the y coordinate field, creating more confusion than already exists.