input: Write a python function to count the frequency of characters in a string using a class.
  assert FrequencyCounter("hello").calculate_frequency() == {'h': 1, 'e': 1, 'l': 2, 'o': 1}
  assert FrequencyCounter("teststring").calculate_frequency() == {'t': 3, 'e': 1, 's': 2, 'r': 1, 'i': 1, 'n': 1, 'g': 1}
  assert FrequencyCounter("frequency").calculate_frequency() == {'f': 1, 'r': 1, 'e': 2, 'q': 1, 'u': 1, 'n': 1, 'c': 1, 'y': 1}
output: class FrequencyCounter:
      def __init__(self, string):
          self.string = string
          
      def calculate_frequency(self):
          frequency = {}
          for char in self.string:
              if char in frequency:
                  frequency[char] += 1
              else:
                  frequency[char] = 1
          return frequency