input: Write a function to compute the average of the elements in a list using a class.
  assert AverageCalculator([3, 4, 5, 6, 7]).get_average() == 5.0
  assert AverageCalculator([1, 2, 3, 4, 5]).get_average() == 3.0
  assert AverageCalculator([10, 20, 30]).get_average() == 20.0
output: class AverageCalculator:
      def __init__(self, numbers):
          self.numbers = numbers
  
      def get_average(self):
          return sum(self.numbers) / len(self.numbers)