input: Write a python function to convert temperature between Celsius, Fahrenheit, and Kelvin.
  assert convert_temperature('C', 'F', 0) == 32
  assert convert_temperature('F', 'C', 32) == 0
  assert convert_temperature('C', 'K', 0) == 273.15
  assert convert_temperature('K', 'C', 273.15) == 0
  assert convert_temperature('F', 'K', 32) == 273.15
  assert convert_temperature('K', 'F', 273.15) == 32
output: def convert_temperature(from_unit, to_unit, value):
      if from_unit == 'C':
          if to_unit == 'F':
              return (value * 9/5) + 32
          elif to_unit == 'K':
              return value + 273.15
      elif from_unit == 'F':
          if to_unit == 'C':
              return (value - 32) * 5/9
          elif to_unit == 'K':
              return (value - 32) * 5/9 + 273.15
      elif from_unit == 'K':
          if to_unit == 'C':
              return value - 273.15
          elif to_unit == 'F':
              return (value - 273.15) * 9/5 + 32
      return None