Custom world generation/position source


 * : The namespaced id of the position source type. Must be one of  and.
 * If  is , additional field is as follow :
 * : The block position.
 * : X.
 * : Y.
 * : Z.
 * If  is , additional fields are as follows :
 * : The int-array UUID of an entity.
 * : (optional, defaults to 0.0) The Y offset from entity's feet.
 * : (optional, defaults to 0.0) The Y offset from entity's feet.
 * : (optional, defaults to 0.0) The Y offset from entity's feet.
 * : (optional, defaults to 0.0) The Y offset from entity's feet.
 * : (optional, defaults to 0.0) The Y offset from entity's feet.