code: def slice_3d_grid(grid, n):
      
      phen_grid = initialize_grid((len(grid[0]), len(grid)), 0)
  
      for i in range(len(grid)):
          for j in range(len(grid[i])):
              phen_grid[i][j] = grid[i][j][n]
  
      return phen_grid
docs: Takes a three dimensional array and an integer (n) and returns a 2d array
      containing the Nth value from the 3rd dimension at each location in the
      grid.