input: Write a python function to calculate the sum of a 2D array.
  assert sum2dArray([[1,2], [3,4], [5,6]]) == 21
  assert sum2dArray([[0,0], [0,0], [0,0]]) == 0
  assert sum2dArray([[-1,-2], [-3,-4], [-5,-6]]) == -21
  assert sum2dArray([[10,20], [30,40], [50,60]]) == 210
output: def sum2dArray(arr):
      return sum(sum(row) for row in arr)