Remove data member Box::grid_ and Box::empty_box_
Remove data member Box::grid_ and Box::empty_box_

Since bdm::Grid is a singleton, storing a reference to it in every box
is wasting memory
empty_box_ was not used anymore

BioDynaMo/biodynamo,BioDynaMo/biodynamo,BioDynaMo/biodynamo,BioDynaMo/biodynamo