hasMidpoint

  Write a method called hasMidpoint that takes three
   integers as parameters and that returns true if one of the numbers is the
   midpoint of the other two and that returns false otherwise.  A number is
   considered the midpoint of two numbers if it appears halfway between them.
   For example, 12 is the midpoint of 10 and 14.  The numbers might be in any
   order, so any one of the three might be the midpoint of the other two.

   The table below shows sample calls and the value that should be returned.

        Method                   Value        Method                  Value
         Call                   Returned       Call                  Returned
        ---------------------------------    ---------------------------------
        hasMidpoint(10, 12, 14)   true       hasMidpoint(14, 10, 12)   true
        hasMidpoint(12, 10, 14)   true       hasMidpoint(2, 10, 6)     true
        hasMidpoint(8, 8, 8)      true       hasMidpoint(25, 10, -5)   true
        hasMidpoint(2, 3, 4)      true       hasMidpoint(2, 3, 5)      false
        hasMidpoint(3, 1, 3)      false      hasMidpoint(2, 4, 5)      false
        hasMidpoint(21, 9, 58)    false      hasMidpoint(2, 8, 16)     false