input: Write a python function to calculate the mean and standard deviation of a numpy array.
  assert np.isclose(calculate_mean_std(np.array([2, 3, 4, 5, 7])), (4.2, 1.6)).all()
  assert np.isclose(calculate_mean_std(np.array([1, 2, 3, 4, 5])), (3.0, 1.41421356237)).all()
  assert np.isclose(calculate_mean_std(np.array([10, 20, 30, 40, 50])), (30.0, 14.1421356237)).all()
output: import numpy as np
  
  def calculate_mean_std(arr):
      mean = np.mean(arr)
      std_dev = np.std(arr)
      return mean, std_dev