Grid: destructors and memory leaks

[casey-c]

Everything in a grid, GridRegions and GridCells, are dynamically allocated but never cleaned up. Deleting the grid object should recursively delete every cell in the root region's array (this should be all the cells that are allocated, since pointers to them are shared between regions, not made anew each time), and also recursively delete the child regions too (making sure not to delete the cells twice).