input: Write a function to create a Python class `SumOfDigits` that takes a given number and calculates the sum of its digits. The class should have an `__init__` method to initialize the number and a method `calculate_sum_of_digits` to compute the sum of the digits. If the input number is `6342`, the output should be `digit_sum.calculate_sum_of_digits()` equals to `15`.
  
  ### Example Output for assert:
  assert SumOfDigits(6342).calculate_sum_of_digits() == 15
  assert SumOfDigits(123).calculate_sum_of_digits() == 6
  assert SumOfDigits(0).calculate_sum_of_digits() == 0
  assert SumOfDigits(987654321).calculate_sum_of_digits() == 45
output: class SumOfDigits:
      def __init__(self, number):
          self.number = number
  
      def calculate_sum_of_digits(self):
          return sum(int(digit) for digit in str(self.number))