input: Write a function to find the minimum element in a list.
  assert find_minimum([34, 48, 19, 61, 3]) == 3
  assert find_minimum([10, 20, 30, 40, 5]) == 5
  assert find_minimum([-1, -5, -10, 0, 1]) == -10
output: def find_minimum(lst):
      if not lst:
          return None
      min_element = lst[0]
      for num in lst:
          if num < min_element:
              min_element = num
      return min_element