input: Write a python function to apply a natural logarithm transformation to each element of a numpy array.
  assert np.allclose(apply_log_transformation(np.array([0.1, 0.4, 0.9, 2.2])), np.log(np.array([0.1, 0.4, 0.9, 2.2])))
  assert np.allclose(apply_log_transformation(np.array([1, 10, 100])), np.log(np.array([1, 10, 100])))
  assert np.allclose(apply_log_transformation(np.array([0.5, 2, 4])), np.log(np.array([0.5, 2, 4])))
output: import numpy as np
  
  def apply_log_transformation(arr):
      return np.log(arr)