input: Write a python function to calculate the mean of a numpy array.
  assert calculate_mean(np.array([1, 4, 6, 5, 9], dtype=np.float64)) == 5.0
  assert calculate_mean(np.array([10, 20, 30, 40, 50], dtype=np.float64)) == 30.0
  assert calculate_mean(np.array([1.5, 2.5, 3.5, 4.5, 5.5], dtype=np.float64)) == 3.5
output: import numpy as np
  
  def calculate_mean(arr):
      return np.mean(arr)