Post Lab 25
  Write a function
  called compute_average(an_integer_list).  This function
  takes a single parameter, a list of integers.  Your function should
  return a floating point value: the arithmetic mean of the integers
  in the list.
Lists
  Write a function
  called compute_average(an_integer_list).  This function
  takes a single parameter, a list of integers.  Your function should
  return a floating point value: the arithmetic mean of the integers
  in the list.