input: Write a function to calculate the sum of all elements in a 2D matrix.
  assert matrixSum([[1, 2, 3], [4, 5, 6]]) == 21
  assert matrixSum([[0, 0, 0], [0, 0, 0]]) == 0
  assert matrixSum([[1, 1, 1], [1, 1, 1]]) == 6
  assert matrixSum([[-1, -2, -3], [-4, -5, -6]]) == -21
output: def matrixSum(matrix):
      return sum(sum(row) for row in matrix)