input: Write a python function to format a string by removing all characters that are not alphanumeric or underscores, and then replace spaces with underscores.
  assert StringFormatter("Hello, World!").format_string() == "Hello_World"
  assert StringFormatter("Python 3.9").format_string() == "Python_39"
  assert StringFormatter("Format_this_string!").format_string() == "Format_this_string"
output: class StringFormatter:
      def __init__(self, string):
          self.string = string
  
      def format_string(self):
          return ''.join(c if c.isalnum() or c == ' ' else '' for c in self.string).replace(' ', '_')