input: Write a function to calculate the square root of a user-defined positive number using Python's math module. If the input number is not positive, return None.
  assert calculate_square_root(100)==10.0
  assert calculate_square_root(0)==None
  assert calculate_square_root(-4)==None
  assert calculate_square_root(9)==3.0
  assert calculate_square_root(2)==1.4142135623730951
output: import math
  
  def calculate_square_root(number):
      if number < 0:
          return None
      return math.sqrt(number)