input: Write a python function to filter lowercase characters from a string.
  assert LowercaseFilter("HelloWorld").filter_string() == "elloorld"
  assert LowercaseFilter("PythonIsFun").filter_string() == "ythonisun"
  assert LowercaseFilter("123abcDEF").filter_string() == "abc"
output: class LowercaseFilter:
      def __init__(self, s):
          self.s = s
  
      def filter_string(self):
          return ''.join([c for c in self.s if c.islower()])