input: Write a python function to find the most frequent character in a given string.
  assert most_frequent_character("Lorem ipsum dolor sit amet consectetur adipiscing elit morbi in ultrices") == (' ', 10)
  assert most_frequent_character("hello world") == ('l', 3)
  assert most_frequent_character("aabbccddeeff") == ('a', 2)
output: def most_frequent_character(s):
      from collections import Counter
      counter = Counter(s)
      most_common = counter.most_common(1)[0]
      return most_common